[codex] Reconcile public status docs

[knhn1004]

Summary

• align public docs around shipped Gemini CLI install/hooks
• mark macOS OS-keychain as shipped while keeping YubiKey/Linux/Windows signer backends on the roadmap
• add a contributor checklist for keeping public status docs synchronized

Validation

• targeted stale-phrase search for old contradictory claims
• git diff --check

Notes

• mkdocs strict build could not run because the local Homebrew Python 3.14 installation fails loading pyexpat/libexpat before pip can install docs requirements.