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/protocol/src/protocol.rs

@@ -1464,6 +1464,9 @@ pub enum EventMsg {
     /// Updated session metadata (e.g., thread name changes).
     ThreadNameUpdated(ThreadNameUpdatedEvent),
 
+    /// Updated long-running goal metadata for the thread.
+    ThreadGoalUpdated(ThreadGoalUpdatedEvent),
+
     /// Incremental MCP startup progress updates.
     McpStartupUpdate(McpStartupUpdateEvent),
 
@@ -3612,6 +3615,43 @@ pub struct ThreadNameUpdatedEvent {
     pub thread_name: Option<String>,
 }
 
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "protocol/")]
+pub enum ThreadGoalStatus {
+    Active,
+    Paused,
+    BudgetLimited,
+    Complete,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "protocol/")]
+pub struct ThreadGoal {
+    pub thread_id: ThreadId,
+    pub objective: String,
+    pub status: ThreadGoalStatus,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub token_budget: Option<i64>,
+    pub tokens_used: i64,
+    pub time_used_seconds: i64,
+    pub created_at: i64,
+    pub updated_at: i64,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "protocol/")]
+pub struct ThreadGoalUpdatedEvent {
+    pub thread_id: ThreadId,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub turn_id: Option<String>,
+    pub goal: ThreadGoal,
+}
+
 /// User's decision in response to an ExecApprovalRequest.
 #[derive(Debug, Default, Clone, Deserialize, Serialize, PartialEq, Eq, Display, JsonSchema, TS)]
 #[serde(rename_all = "snake_case")]
@@ -3714,6 +3754,7 @@ pub enum TurnAbortReason {
     Interrupted,
     Replaced,
     ReviewEnded,
+    BudgetLimited,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]