feat: add thread lifecycle contributor hooks (#22476)

## Why

Extensions that need thread-scoped state currently only get a start-time
callback. That is enough for seeding stores, but it leaves the host
without a shared extension seam for later thread rehydrate and flush
work as thread ownership evolves. This PR turns that start-only seam
into a host-owned thread lifecycle contributor contract so
extension-private state can stay behind the extension API instead of
leaking extra orchestration through core.

## What changed

- Replaced `ThreadStartContributor` with `ThreadLifecycleContributor`
and added typed lifecycle inputs for thread start, resume, and stop. The
contract lives in
[`contributors/thread_lifecycle.rs`](https://github.com/openai/codex/blob/d0e9211f70e58d6b07ef07e84f359d1b9aa25955/codex-rs/ext/extension-api/src/contributors/thread_lifecycle.rs#L1-L64).
- Kept the existing start-time behavior intact by routing session
construction through `on_thread_start`.
- Invoked `on_thread_stop` during session shutdown before thread-scoped
extension state is dropped, while isolating contributor failures behind
warning logs.
- Migrated `git-attribution` and `guardian` onto the lifecycle
registration path.
- Renamed the extension registry plumbing from start-specific
contributors to lifecycle-specific contributors.

## Notes

`on_thread_resume` is introduced at the API boundary here so extensions
can target the final lifecycle shape; host resume dispatch can be wired
where that runtime path is finalized.

codex-rs/core/src/codex_thread.rs

@@ -140,6 +140,22 @@ impl CodexThread {
         self.codex.session_loop_termination.clone().await;
     }
 
+    pub(crate) fn emit_thread_resume_lifecycle(&self) {
+        for contributor in self
+            .codex
+            .session
+            .services
+            .extensions
+            .thread_lifecycle_contributors()
+        {
+            contributor.on_thread_resume(codex_extension_api::ThreadResumeInput {
+                thread_id: self.codex.session.conversation_id,
+                session_store: &self.codex.session.services.session_extension_data,
+                thread_store: &self.codex.session.services.thread_extension_data,
+            });
+        }
+    }
+
     pub async fn apply_goal_resume_runtime_effects(&self) -> anyhow::Result<()> {
         self.codex
             .session