docs: add Codex Desktop coordination skill

[SSBrouhard]

What Changed

• Added the agent-only firstmate-codexapp skill with OpenAI agent metadata for coordinating Codex Desktop work.
• Documented Codex Desktop/App backend boundaries across the firstmate instructions and project docs.
• Kept the surrounding README, contributing, architecture, and configuration references aligned with the new boundary language.

Risk Assessment

✅ Low: Captain, the change is limited to documentation and an internal skill manifest/playbook with no runtime behavior changes found.

Testing

Completed 1 recorded test check.

Pipeline

Updates from git push no-mistakes

⏭️ **intent** - skipped

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

✅ **Review** - passed

✅ No issues found.

✅ **Test** - passed

✅ No issues found.

• command -v tmux >/dev/null || { echo "tmux is required for e2e tests" >&2; exit 1; }; tmux -V; rc=0; for t in tests/*.test.sh; do echo "== $t =="; bash "$t" || rc=1; done; exit "$rc"

✅ **Document** - passed

✅ No issues found.

✅ **Lint** - passed

✅ No issues found.

✅ **Push** - passed

✅ No issues found.