feat(diagnostics): warn when project .codex/config.toml bypasses OpenCodex (cherry-pick PR #70)

Cherry-pick of 98d227d + b19384c from feat/project-codex-config-warnings.
All 5 review feedback items addressed by contributor. Additional fix:
- Test isolation: set CODEX_HOME in project-config-warnings.test.ts to prevent
  reading real user config during tests
- Fix cache test to use collectProjectCodexConfigWarnings with explicit cwd

Co-authored-by: Wibias <37517432+Wibias@users.noreply.github.com>