[Flow] Clarify MCP server mode usage and stability — Advanced

[github-actions]

Summary

codex mcp (server mode) is marked experimental. Clarify supported capabilities, example client flows (Inspector), and caveats to set expectations.

Current vs Expected

• Current: Brief mention in codex-rs/README.
• Expected: A small section in docs/advanced.md with: command examples, tools/list output expectations, compatibility notes.

Proposed Changes

• Expand docs/advanced.md with a “Codex as MCP server” section.
• Provide Inspector example and expected tool schema.
• Note limitations and where to file feedback.

Acceptance Criteria

• Docs updated with working example and screenshots/log snippets.
• Linked from codex-rs/README MCP section.

References

• codex-rs/README.md (MCP section), docs/advanced.md