[firstmate#275] docs: add Codex Desktop coordination skill

[github-actions]

Decision needed - firstmate#275

PR review by SSBrouhard · merge-ready

docs: add Codex Desktop coordination skill

Situation

• Compliance: pass
• Tests: green
• Notes: compliance=pass tests=green

Triage

• Summary: Adds an agent-only firstmate-codexapp skill and OpenAI agent manifest, and updates AGENTS.md/CONTRIBUTING.md/README.md/docs to document that Codex Desktop visible threads are a coordinated companion workflow, not a selectable FM_BACKEND value.
• Product implications: Routine internal documentation and skill-manifest work with no runtime code, config parsing, or executable behavior changes; the new skill/docs are consistent with existing conventions (verified no docs/codex-app-backend.md or conflicting codex-app backend references exist in the checkout), so this is low-risk maintenance rather than something needing owner discussion.
• Recommended next step: merge - documentation-only change, internally consistent, no behavior or security impact found in target-src

Recommended action

Merge - compliance and tests are green.

Your decision

Tick one box for a quick call, or reply with a slash-command (/merge, /close, /decline <reason>, /hold, /comment <text>, /request-changes <text>):

• Merge it
• Close / decline
• Investigate - deep code-grounded review (leaves this card open)
• Hold - I'll handle this manually

_{Only the repository owner can drive this decision - everyone else's edits and comments are ignored.}