start of hooks engine (#13276)

(Experimental)

This PR adds a first MVP for hooks, with SessionStart and Stop

The core design is:

- hooks live in a dedicated engine under codex-rs/hooks
- each hook type has its own event-specific file
- hook execution is synchronous and blocks normal turn progression while
running
- matching hooks run in parallel, then their results are aggregated into
a normalized HookRunSummary

On the AppServer side, hooks are exposed as operational metadata rather
than transcript-native items:

- new live notifications: hook/started, hook/completed
- persisted/replayed hook results live on Turn.hookRuns
- we intentionally did not add hook-specific ThreadItem variants

Hooks messages are not persisted, they remain ephemeral. The context
changes they add are (they get appended to the user's prompt)

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

@@ -9702,6 +9702,133 @@ async fn final_reasoning_then_message_without_deltas_are_rendered() {
     assert_snapshot!(combined);
 }
 
+#[tokio::test]
+async fn deltas_then_same_final_message_are_rendered_snapshot() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+
+    // Stream some reasoning deltas first.
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "I will ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "first analyze the ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
+            delta: "request.".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentReasoning(AgentReasoningEvent {
+            text: "request.".into(),
+        }),
+    });
+
+    // Then stream answer deltas, followed by the exact same final message.
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
+            delta: "Here is the ".into(),
+        }),
+    });
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
+            delta: "result.".into(),
+        }),
+    });
+
+    chat.handle_codex_event(Event {
+        id: "s1".into(),
+        msg: EventMsg::AgentMessage(AgentMessageEvent {
+            message: "Here is the result.".into(),
+            phase: None,
+        }),
+    });
+
+    // Snapshot the combined visible content to ensure we render as expected
+    // when deltas are followed by the identical final message.
+    let cells = drain_insert_history(&mut rx);
+    let combined = cells
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<String>();
+    assert_snapshot!(combined);
+}
+
+#[tokio::test]
+async fn hook_events_render_snapshot() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+
+    chat.handle_codex_event(Event {
+        id: "hook-1".into(),
+        msg: EventMsg::HookStarted(codex_protocol::protocol::HookStartedEvent {
+            turn_id: None,
+            run: codex_protocol::protocol::HookRunSummary {
+                id: "session-start:0:/tmp/hooks.json".to_string(),
+                event_name: codex_protocol::protocol::HookEventName::SessionStart,
+                handler_type: codex_protocol::protocol::HookHandlerType::Command,
+                execution_mode: codex_protocol::protocol::HookExecutionMode::Sync,
+                scope: codex_protocol::protocol::HookScope::Thread,
+                source_path: PathBuf::from("/tmp/hooks.json"),
+                display_order: 0,
+                status: codex_protocol::protocol::HookRunStatus::Running,
+                status_message: Some("warming the shell".to_string()),
+                started_at: 1,
+                completed_at: None,
+                duration_ms: None,
+                entries: vec![],
+            },
+        }),
+    });
+
+    chat.handle_codex_event(Event {
+        id: "hook-1".into(),
+        msg: EventMsg::HookCompleted(codex_protocol::protocol::HookCompletedEvent {
+            turn_id: None,
+            run: codex_protocol::protocol::HookRunSummary {
+                id: "session-start:0:/tmp/hooks.json".to_string(),
+                event_name: codex_protocol::protocol::HookEventName::SessionStart,
+                handler_type: codex_protocol::protocol::HookHandlerType::Command,
+                execution_mode: codex_protocol::protocol::HookExecutionMode::Sync,
+                scope: codex_protocol::protocol::HookScope::Thread,
+                source_path: PathBuf::from("/tmp/hooks.json"),
+                display_order: 0,
+                status: codex_protocol::protocol::HookRunStatus::Completed,
+                status_message: Some("warming the shell".to_string()),
+                started_at: 1,
+                completed_at: Some(11),
+                duration_ms: Some(10),
+                entries: vec![
+                    codex_protocol::protocol::HookOutputEntry {
+                        kind: codex_protocol::protocol::HookOutputEntryKind::Warning,
+                        text: "Heads up from the hook".to_string(),
+                    },
+                    codex_protocol::protocol::HookOutputEntry {
+                        kind: codex_protocol::protocol::HookOutputEntryKind::Context,
+                        text: "Remember the startup checklist.".to_string(),
+                    },
+                ],
+            },
+        }),
+    });
+
+    let cells = drain_insert_history(&mut rx);
+    let combined = cells
+        .iter()
+        .map(|lines| lines_to_single_string(lines))
+        .collect::<String>();
+    assert_snapshot!("hook_events_render_snapshot", combined);
+}
+
 // Combined visual snapshot using vt100 for history + direct buffer overlay for UI.
 // This renders the final visual as seen in a terminal: history above, then a blank line,
 // then the exec block, another blank line, the status line, a blank line, and the composer.