docs: clarify Codex App backend contract

[SSBrouhard]

What Changed

• Captain, makes docs/codex-app-backend.md the authoritative contract for Codex App backend behavior.
• Clarifies that Codex App is not an accepted runtime backend today and documents the Desktop bridge blocker.
• Reconciles the backend contract language after rebasing the docs onto main.

Risk Assessment

✅ Low: Captain, the net branch diff is documentation-only, narrowly scoped to the Codex App backend contract, and it stays consistent with the existing blocked-backend enforcement.

Testing

Completed 1 recorded test check.

Pipeline

Updates from git push no-mistakes

⏭️ **intent** - skipped

✅ No issues found.

🔧 **Rebase** - 3 issues found → auto-fixed ✅

• ⚠️ docs/architecture.md - merge conflict rebasing onto origin/main
• ⚠️ docs/codex-app-backend.md - merge conflict rebasing onto origin/main
• ⚠️ docs/configuration.md - merge conflict rebasing onto origin/main

🔧 Fix applied.
✅ Re-checked - no issues remain.

✅ **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.

[SSBrouhard]

Closing as duplicate. Original PR #222 is already merged.