[codex] Move thread naming to app server (#21260)

## Why

Thread names are app-server metadata now, backed by the thread store and
sqlite state database. Keeping a core `SetThreadName` op plus a rollout
`thread_name_updated` event made rename persistence live in the wrong
layer and required historical replay support for an event that new
app-server flows should not write.

## What changed

- Removed `Op::SetThreadName` and `EventMsg::ThreadNameUpdated` from the
core protocol and deleted the core handler path that appended rename
events to rollouts.
- Updated app-server `thread/name/set` so both loaded and unloaded
threads write through thread-store metadata and app-server emits
`thread/name/updated` notifications.
- Updated local thread-store name metadata updates to write sqlite title
metadata and the legacy thread-name index without appending rollout
events.
- Removed state extraction and rollout handling for the deleted
thread-name event.

## Validation

- `cargo test -p codex-app-server thread_name_updated_broadcasts`
- `cargo test -p codex-app-server
thread_name_set_is_reflected_in_read_list_and_resume`
- `cargo test -p codex-thread-store
update_thread_metadata_sets_name_on_active_rollout_and_indexes_name`
- `cargo test -p codex-state`
- `cargo check -p codex-mcp-server -p codex-rollout-trace`
- `just fix -p codex-app-server -p codex-thread-store -p codex-state -p
codex-mcp-server -p codex-rollout-trace`

## Docs

No external documentation update is expected for this internal ownership
change.

codex-rs/protocol/src/protocol.rs

@@ -766,11 +766,6 @@ pub enum Op {
     /// to generate a summary which will be returned as an AgentMessage event.
     Compact,
 
-    /// Set a user-facing thread name in the persisted rollout metadata.
-    /// This is a local-only operation handled by codex-core; it does not
-    /// involve the model.
-    SetThreadName { name: String },
-
     /// Set whether the thread remains eligible for memory generation.
     ///
     /// This persists thread-level memory mode metadata without involving the
@@ -903,7 +898,6 @@ impl Op {
             Self::ReloadUserConfig => "reload_user_config",
             Self::ListSkills { .. } => "list_skills",
             Self::Compact => "compact",
-            Self::SetThreadName { .. } => "set_thread_name",
             Self::SetThreadMemoryMode { .. } => "set_thread_memory_mode",
             Self::ThreadRollback { .. } => "thread_rollback",
             Self::Review { .. } => "review",
@@ -1373,9 +1367,6 @@ pub enum EventMsg {
     /// Ack the client's configure message.
     SessionConfigured(SessionConfiguredEvent),
 
-    /// Updated session metadata (e.g., thread name changes).
-    ThreadNameUpdated(ThreadNameUpdatedEvent),
-
     /// Updated long-running goal metadata for the thread.
     ThreadGoalUpdated(ThreadGoalUpdatedEvent),
 
@@ -3592,14 +3583,6 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
     }
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct ThreadNameUpdatedEvent {
-    pub thread_id: ThreadId,
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    #[ts(optional)]
-    pub thread_name: Option<String>,
-}
-
 #[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "protocol/")]