feat(task-state): add transitional SQLite mutex

[unohee]

Summary

• Acquire a kernel-owned SQLite writer lock before the legacy task-state dot-lock.
• Keep dot-lock compatibility for older CLI clients during the transition release.
• Add a regression test proving a dual-lock writer waits for a dot-lock-only client.

Verification

• npm run lint
• npm run typecheck
• npm run build
• npm test -- --run src/taskState/store.test.ts src/taskState/storeLockCompatibility.test.ts