feat(lastcode): safeguard persistent threads

[lastobelus]

Persistent recovery threads can currently be archived or deleted without noticing that checkpoint automation still targets them.

This adds one environment-level persistent-thread designation, exposes it in web and desktop context menus, marks the thread with an italic title and message-square-lock icon across web, desktop, and mobile lists, and blocks archive/delete in both the UI and server until persistence is transferred or disabled. The checkpoint dashboard now detects missing or unprotected configured IDs and can atomically repair its supervisor configuration with --repair-persistent-thread.

Validation:

• Independent LastCode Quick CI passed for dbbdf0e2127696327bcb240e1daf714d342ac368
• 93 focused tests passed
• Contracts, client runtime, server, web, desktop, and mobile typechecks passed
• Changed-file lint, formatting, and diff checks passed
• Real web client verified the lock icon, italic title, persistence toggle, and blocked archive/delete labels
• Automated screenshot capture was attempted, but the shared preview snapshot bridge failed; DOM-level client verification completed successfully

Implemented with GPT-5.6 Sol through the Codex harness.