Part 1 of guardian as extension. This bind all the logic to spawn
another agent from an extension and it adds `ThreadId` in the start
thread collaborator
## Why
Git commit attribution is prompt policy, not session orchestration.
After #21737 adds the extension-registry seam, this moves that
prompt-only behavior out of `codex-core` so `Session` can consume
extension-contributed prompt fragments instead of owning a one-off
policy path itself.
Before this PR, `Session` injected the trailer instruction directly from
`codex-core` ([session
assembly](a57a747eb6/codex-rs/core/src/session/mod.rs (L2733-L2739)),
[helper
module](a57a747eb6/codex-rs/core/src/commit_attribution.rs (L1-L33))).
This branch moves that same responsibility into
[`codex-git-attribution`](b5029a6736/codex-rs/ext/git-attribution/src/lib.rs (L14-L100)).
## What changed
- Added the `codex-git-attribution` extension crate.
- Snapshot `CodexGitCommit` plus `commit_attribution` at thread start,
then contribute the developer-policy fragment through the extension
registry.
- Register the extension in app-server thread extensions.
- Remove the old `codex-core` helper module and direct `Session`
injection path.
This keeps the existing behavior intact: the prompt is only contributed
when `CodexGitCommit` is enabled, blank attribution still disables the
trailer, and the default remains `Codex <noreply@openai.com>`.
## Stack
- Stacked on #21737.
