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/Cargo.lock

@@ -2918,7 +2918,6 @@ dependencies = [
  "codex-core",
  "codex-extension-api",
  "codex-features",
- "codex-protocol",
  "pretty_assertions",
 ]