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-protocol/schema/json/v2/ThreadGoalClearedNotification.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "threadId"
+  ],
+  "title": "ThreadGoalClearedNotification",
+  "type": "object"
+}