Add exact-target navigation and optional Codex provenance

[Traviinam]

Summary

• add exact opaque-target Worktree and Changed File navigation actions
• revalidate targets and host context immediately before native actions
• classify Codex provenance only from stable explicit evidence and fail closed on conflicts
• route exact current Codex task or project restoration through proven host capability
• keep Open File in Codex unavailable until the host exposes a safe exact-file navigation API
• share native-action labels and result presentation across UI surfaces

Verification

• full test suite: 266 passed
• TypeScript typecheck
• ESLint
• Prettier
• production build
• git diff check

Closes #15