Persist Daybreak preferences in thread metadata (#42854)

## Why

Clients need a per-thread Daybreak toggle that survives restarts and can be
restored on resume without changing the access program of an active or future
turn.

## What changed

- Add experimental `daybreakEnabled` support to `thread/metadata/update` and
  expose the saved value on thread responses.
- Store the preference in SQLite, including for threads without a first turn,
  and preserve it through reads, resumes, reconciliation, and restarts.
- Copy the preference when forking a thread while keeping subsequent updates
  independent.
- Keep `daybreakEnabled` separate from the per-turn `cyberAccessProgram` field;
  app-server does not derive one from the other.

## Testing

- Cover persistence across restarts, cold reads, reconciliation, and forks.
- Verify metadata updates do not alter the access program of active or later
  turns.

GitOrigin-RevId: bcf987a9fcd7b0ac8a88b1c8f0df06b33ed8ad6e

codex-rs/app-server-protocol/src/protocol/v2/thread_data.rs

@@ -283,6 +283,9 @@ pub struct Thread {
     pub git_info: Option<GitInfo>,
     /// Optional user-facing thread title.
     pub name: Option<String>,
+    /// Saved Daybreak choice, independent of turn execution. Null if unset.
+    #[experimental("thread.daybreakEnabled")]
+    pub daybreak_enabled: Option<bool>,
     /// Only populated on `thread/resume`, `thread/rollback`, `thread/fork`, and `thread/read`
     /// (when `includeTurns` is true) responses.
     /// For all other responses and notifications returning a Thread,
@@ -330,6 +333,7 @@ struct ThreadCompatibility {
     agent_role: Option<String>,
     git_info: Option<GitInfo>,
     name: Option<String>,
+    daybreak_enabled: Option<bool>,
     turns: Vec<Turn>,
 }
 
@@ -370,6 +374,7 @@ impl<'de> Deserialize<'de> for Thread {
             agent_role: thread.agent_role,
             git_info: thread.git_info,
             name: thread.name,
+            daybreak_enabled: thread.daybreak_enabled,
             turns: thread.turns,
         })
     }