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/core/src/session/turn.rs

@@ -1502,6 +1502,7 @@ pub(super) fn realtime_text_for_event(msg: &EventMsg) -> Option<String> {
         | EventMsg::AgentReasoningSectionBreak(_)
         | EventMsg::SessionConfigured(_)
         | EventMsg::ThreadNameUpdated(_)
+        | EventMsg::ThreadGoalUpdated(_)
         | EventMsg::McpStartupUpdate(_)
         | EventMsg::McpStartupComplete(_)
         | EventMsg::McpToolCallBegin(_)