Add goal app-server API (2 / 5) (#18074)

Adds the app-server v2 goal API on top of the persisted goal state from
PR 1.

## Why

Clients need a stable app-server surface for reading and controlling
materialized thread goals before the model tools and TUI can use them.
Goal changes also need to be observable by app-server clients, including
clients that resume an existing thread.

## What changed

- Added v2 `thread/goal/get`, `thread/goal/set`, and `thread/goal/clear`
RPCs for materialized threads.
- Added `thread/goal/updated` and `thread/goal/cleared` notifications so
clients can keep local goal state in sync.
- Added resume/snapshot wiring so reconnecting clients see the current
goal state for a thread.
- Added app-server handlers that reconcile persisted rollout state
before direct goal mutations.
- Updated the app-server README plus generated JSON and TypeScript
schema fixtures for the new API surface.

## Verification

- Added app-server v2 coverage for goal get/set/clear behavior,
notification emission, resume snapshots, and non-local thread-store
interactions.

codex-rs/app-server/src/thread_state.rs

@@ -1,6 +1,7 @@
 use crate::outgoing_message::ConnectionId;
 use crate::outgoing_message::ConnectionRequestId;
 use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ThreadGoal;
 use codex_app_server_protocol::ThreadHistoryBuilder;
 use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnError;
@@ -9,6 +10,7 @@ use codex_core::ThreadConfigSnapshot;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::RolloutItem;
+use codex_rollout::state_db::StateDbHandle;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::collections::HashMap;
 use std::collections::HashSet;
@@ -31,6 +33,8 @@ pub(crate) struct PendingThreadResumeRequest {
     pub(crate) config_snapshot: ThreadConfigSnapshot,
     pub(crate) instruction_sources: Vec<AbsolutePathBuf>,
     pub(crate) thread_summary: codex_app_server_protocol::Thread,
+    pub(crate) emit_thread_goal_update: bool,
+    pub(crate) thread_goal_state_db: Option<StateDbHandle>,
     pub(crate) include_turns: bool,
 }
 
@@ -38,6 +42,16 @@ pub(crate) struct PendingThreadResumeRequest {
 pub(crate) enum ThreadListenerCommand {
     // SendThreadResumeResponse is used to resume an already running thread by sending the thread's history to the client and atomically subscribing for new updates.
     SendThreadResumeResponse(Box<PendingThreadResumeRequest>),
+    // EmitThreadGoalUpdated is used to order app-server goal updates with running-thread resume responses.
+    EmitThreadGoalUpdated {
+        goal: ThreadGoal,
+    },
+    // EmitThreadGoalCleared is used to order app-server goal clears with running-thread resume responses.
+    EmitThreadGoalCleared,
+    // EmitThreadGoalSnapshot is used to read and emit the latest goal state in the listener order.
+    EmitThreadGoalSnapshot {
+        state_db: StateDbHandle,
+    },
     // ResolveServerRequest is used to notify the client that the request has been resolved.
     // It is executed in the thread listener's context to ensure that the resolved notification is ordered with regard to the request itself.
     ResolveServerRequest {