v3.12.3: Atomic RVF export + self-healing stores

What's New

Fixes a bug that could silently switch off vector search — permanently. If an
export of the RVF store was interrupted (a hook timing out, a session ending, a
hard kill), it left behind a store file AQE could neither open nor replace — so
every later run quietly fell back to SQLite, and only deleting the file by hand
brought it back. One reporting project hit this three times in a day; this
repository turned out to have been running degraded for nine days.

If you're affected, you don't need to do anything. The next run detects the
unusable store, sets it aside (kept, not deleted), and rebuilds the cache from
memory.db.

Fixed (#563)

• Brain exports are now atomic — written to a temp file and moved into place
  only once complete, so an interruption leaves your previous store untouched.
• Unusable stores are quarantined and rebuilt automatically instead of
  silently disabling vector search. Covers brain.rvf, patterns.rvf, and the
  pattern store.
• RVF fallbacks now log why instead of failing silently.
• Lock cleanup no longer risks a live process's store.

Added (#562)

• Flywheel receipts are signed with the Cognitum platform identity, chaining
  all QE evidence to one externally verifiable key.

Thanks to @pacphi for the detailed report.

Getting Started

```bash
npx agentic-qe init --auto
```

See CHANGELOG for full details.