Fix Codex annotate-last message selection

[mu-hashmi]

Summary

• make Codex annotate-last ignore assistant messages from the currently active turn
• add a regression test for status/preamble messages shadowing the previous response
• document that plannotator-last skills should avoid pre-command preambles

Fixes #739.

Tests

• bun run typecheck
• bun test apps/hook/server/codex-session.test.ts scripts/install.test.ts
• git diff --check

[backnotprop]

Thanks, merging.

Also for codex (cli), i usually just do !plannotator last

But in the app i know !commands dont work.

Just as sharing in case you use cli