fix: detect Codex runtime in desktop app

[SaladDay]

What changed

• Treat a non-empty CODEX_THREAD_ID as an active Codex runtime.
• Preserve the existing PATH-based detection for all coding agents.
• Cover runtime-only, PATH merge, missing, empty, and whitespace marker cases.
• Add the repository blind-review workflow used for this change.

Scope

This PR only addresses Codex running inside ChatGPT Desktop. It does not add runtime detection for other agents or change installation and configuration behavior.

Validation

• 500 tests passed.
• Type check passed.
• Production build passed.
• Lint passed with one existing warning in test/sdk/speech.test.ts.
• Final blind review found no issues.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}