Plan mode: stream proposed plans, emit plan items, and render in TUI (#9786)

## Summary
- Stream proposed plans in Plan Mode using `<proposed_plan>` tags parsed
in core, emitting plan deltas plus a plan `ThreadItem`, while stripping
tags from normal assistant output.
- Persist plan items and rebuild them on resume so proposed plans show
in thread history.
- Wire plan items/deltas through app-server protocol v2 and render a
dedicated proposed-plan view in the TUI, including the “Implement this
plan?” prompt only when a plan item is present.

## Changes

### Core (`codex-rs/core`)
- Added a generic, line-based tag parser that buffers each line until it
can disprove a tag prefix; implements auto-close on `finish()` for
unterminated tags. `codex-rs/core/src/tagged_block_parser.rs`
- Refactored proposed plan parsing to wrap the generic parser.
`codex-rs/core/src/proposed_plan_parser.rs`
- In plan mode, stream assistant deltas as:
  - **Normal text** → `AgentMessageContentDelta`
  - **Plan text** → `PlanDelta` + `TurnItem::Plan` start/completion  
  (`codex-rs/core/src/codex.rs`)
- Final plan item content is derived from the completed assistant
message (authoritative), not necessarily the concatenated deltas.
- Strips `<proposed_plan>` blocks from assistant text in plan mode so
tags don’t appear in normal messages.
(`codex-rs/core/src/stream_events_utils.rs`)
- Persist `ItemCompleted` events only for plan items for rollout replay.
(`codex-rs/core/src/rollout/policy.rs`)
- Guard `update_plan` tool in Plan Mode with a clear error message.
(`codex-rs/core/src/tools/handlers/plan.rs`)
- Updated Plan Mode prompt to:  
  - keep `<proposed_plan>` out of non-final reasoning/preambles  
  - require exact tag formatting  
  - allow only one `<proposed_plan>` block per turn  
  (`codex-rs/core/templates/collaboration_mode/plan.md`)

### Protocol / App-server protocol
- Added `TurnItem::Plan` and `PlanDeltaEvent` to core protocol items.
(`codex-rs/protocol/src/items.rs`, `codex-rs/protocol/src/protocol.rs`)
- Added v2 `ThreadItem::Plan` and `PlanDeltaNotification` with
EXPERIMENTAL markers and note that deltas may not match the final plan
item. (`codex-rs/app-server-protocol/src/protocol/v2.rs`)
- Added plan delta route in app-server protocol common mapping.
(`codex-rs/app-server-protocol/src/protocol/common.rs`)
- Rebuild plan items from persisted `ItemCompleted` events on resume.
(`codex-rs/app-server-protocol/src/protocol/thread_history.rs`)

### App-server
- Forward plan deltas to v2 clients and map core plan items to v2 plan
items. (`codex-rs/app-server/src/bespoke_event_handling.rs`,
`codex-rs/app-server/src/codex_message_processor.rs`)
- Added v2 plan item tests.
(`codex-rs/app-server/tests/suite/v2/plan_item.rs`)

### TUI
- Added a dedicated proposed plan history cell with special background
and padding, and moved “• Proposed Plan” outside the highlighted block.
(`codex-rs/tui/src/history_cell.rs`, `codex-rs/tui/src/style.rs`)
- Only show “Implement this plan?” when a plan item exists.
(`codex-rs/tui/src/chatwidget.rs`,
`codex-rs/tui/src/chatwidget/tests.rs`)

<img width="831" height="847" alt="Screenshot 2026-01-29 at 7 06 24 PM"
src="https://github.com/user-attachments/assets/69794c8c-f96b-4d36-92ef-c1f5c3a8f286"
/>

### Docs / Misc
- Updated protocol docs to mention plan deltas.
(`codex-rs/docs/protocol_v1.md`)
- Minor plumbing updates in exec/debug clients to tolerate plan deltas.
(`codex-rs/debug-client/src/reader.rs`, `codex-rs/exec/...`)

## Tests
- Added core integration tests:
  - Plan mode strips plan from agent messages.
  - Missing `</proposed_plan>` closes at end-of-message.  
  (`codex-rs/core/tests/suite/items.rs`)
- Added unit tests for generic tag parser (prefix buffering, non-tag
lines, auto-close). (`codex-rs/core/src/tagged_block_parser.rs`)
- Existing app-server plan item tests in v2.
(`codex-rs/app-server/tests/suite/v2/plan_item.rs`)

## Notes / Behavior
- Plan output no longer appears in standard assistant text in Plan Mode;
it streams via `PlanDelta` and completes as a `TurnItem::Plan`.
- The final plan item content is authoritative and may diverge from
streamed deltas (documented as experimental).
- Reasoning summaries are not filtered; prompt instructs the model not
to include `<proposed_plan>` outside the final plan message.

## Codex Author
`codex fork 019bec2d-b09d-7450-b292-d7bcdddcdbfb`

codex-rs/tui/src/chatwidget/tests.rs

@@ -809,6 +809,7 @@ async fn make_chatwidget_manual(
         rate_limit_switch_prompt: RateLimitSwitchPromptState::default(),
         rate_limit_poller: None,
         stream_controller: None,
+        plan_stream_controller: None,
         running_commands: HashMap::new(),
         suppressed_exec_calls: HashSet::new(),
         skills_all: Vec::new(),
@@ -840,6 +841,9 @@ async fn make_chatwidget_manual(
         needs_final_message_separator: false,
         had_work_activity: false,
         saw_plan_update_this_turn: false,
+        saw_plan_item_this_turn: false,
+        plan_delta_buffer: String::new(),
+        plan_item_active: false,
         last_separator_elapsed_secs: None,
         last_rendered_width: std::cell::Cell::new(None),
         feedback: codex_feedback::CodexFeedback::new(),
@@ -1277,7 +1281,7 @@ async fn plan_implementation_popup_skips_when_messages_queued() {
 }
 
 #[tokio::test]
-async fn plan_implementation_popup_shows_on_plan_update_without_message() {
+async fn plan_implementation_popup_skips_without_proposed_plan() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5")).await;
     chat.set_feature_enabled(Feature::CollaborationModes, true);
     let plan_mask =
@@ -1295,10 +1299,31 @@ async fn plan_implementation_popup_shows_on_plan_update_without_message() {
     });
     chat.on_task_complete(None, false);
 
+    let popup = render_bottom_popup(&chat, 80);
+    assert!(
+        !popup.contains(PLAN_IMPLEMENTATION_TITLE),
+        "expected no plan popup without proposed plan output, got {popup:?}"
+    );
+}
+
+#[tokio::test]
+async fn plan_implementation_popup_shows_after_proposed_plan_output() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(Some("gpt-5")).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+    let plan_mask =
+        collaboration_modes::mask_for_kind(chat.models_manager.as_ref(), ModeKind::Plan)
+            .expect("expected plan collaboration mask");
+    chat.set_collaboration_mask(plan_mask);
+
+    chat.on_task_started();
+    chat.on_plan_delta("- Step 1\n- Step 2\n".to_string());
+    chat.on_plan_item_completed("- Step 1\n- Step 2\n".to_string());
+    chat.on_task_complete(None, false);
+
     let popup = render_bottom_popup(&chat, 80);
     assert!(
         popup.contains(PLAN_IMPLEMENTATION_TITLE),
-        "expected plan popup after plan update, got {popup:?}"
+        "expected plan popup after proposed plan output, got {popup:?}"
     );
 }
 
@@ -1957,6 +1982,7 @@ async fn unified_exec_wait_after_final_agent_message_snapshot() {
         id: "turn-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -1991,6 +2017,7 @@ async fn unified_exec_wait_before_streamed_agent_message_snapshot() {
         id: "turn-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -2779,6 +2806,7 @@ async fn interrupted_turn_error_message_snapshot() {
         id: "task-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -3793,6 +3821,7 @@ async fn interrupt_clears_unified_exec_wait_streak_snapshot() {
         id: "turn-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -3866,6 +3895,7 @@ async fn ui_snapshots_small_heights_task_running() {
         id: "task-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     chat.handle_codex_event(Event {
@@ -3897,6 +3927,7 @@ async fn status_widget_and_approval_modal_snapshot() {
         id: "task-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     // Provide a deterministic header for the status line.
@@ -3949,6 +3980,7 @@ async fn status_widget_active_snapshot() {
         id: "task-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     // Provide a deterministic header via a bold reasoning chunk.
@@ -3998,6 +4030,7 @@ async fn mcp_startup_complete_does_not_clear_running_task() {
         id: "task-1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -4554,6 +4587,7 @@ async fn stream_recovery_restores_previous_status_header() {
         id: "task".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     drain_insert_history(&mut rx);
@@ -4591,6 +4625,7 @@ async fn multiple_agent_messages_in_single_turn_emit_multiple_headers() {
         id: "s1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
 
@@ -4785,6 +4820,7 @@ async fn chatwidget_exec_and_status_layout_vt100_snapshot() {
         id: "t1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     chat.handle_codex_event(Event {
@@ -4832,6 +4868,7 @@ async fn chatwidget_markdown_code_blocks_vt100_snapshot() {
         id: "t1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     // Build a vt100 visual from the history insertions only (no UI overlay)
@@ -4921,6 +4958,7 @@ async fn chatwidget_tall() {
         id: "t1".into(),
         msg: EventMsg::TurnStarted(TurnStartedEvent {
             model_context_window: None,
+            collaboration_mode_kind: ModeKind::Custom,
         }),
     });
     for i in 0..30 {