Harden worker concurrency foundations

[danii1]

Summary

Extracts the low-risk foundations from #47 without introducing its scheduler, worktree lanes, or per-repository concurrency model.

• centralize SQLite connection hardening for shared worker state
• enable WAL, NORMAL synchronous mode, and a 5-second busy timeout on writable connections
• configure read-only connections without mutating persistent database state
• add an awaited and bounded worker shutdown hook for a future execution supervisor
• force a non-zero exit when a second shutdown signal arrives
• continue final cleanup when individual cleanup stages fail

This PR does not enable concurrent task execution. Scheduling and sandbox lifecycle design will follow separately around task-lifetime microVMs and one canonical Git workflow.

Verification

• pre-commit format, lint, and typecheck hooks passed
• pre-push monorepo tests and typecheck passed
• @getdevintern/code: 1313 tests passed
• focused concurrency and shutdown suite: 111 tests passed

Supersedes #47.