Sandbox docs omit .agents from protected metadata paths

What happened?

Some sandbox docs list protected metadata paths under writable roots but omit .agents.

Examples:

• codex-rs/core/README.md says workspace-write keeps .git, resolved gitdir:, and .codex read-only.
• codex-rs/linux-sandbox/README.md gives the same examples for bubblewrap.

The implementation includes .agents in PROTECTED_METADATA_PATH_NAMES, and tests cover .git, .agents, and .codex being blocked under writable roots. The Linux bwrap module comment also mentions .agents alongside .git and .codex.

Expected behavior

The sandbox docs should include .agents in the protected metadata path examples so the documented behavior matches the current filesystem policy.

Additional context

I searched open issues and PRs for terms including .agents, sandbox README/docs, protected metadata, .git, and .codex, and did not find an existing open item that appears to cover this documentation omission.