fix(db): make WAL setup concurrency-safe across packages - #206

Summary

• arm SQLite's 5s busy handler when opening memory, agent-manager, and task-manager connections
• skip the WAL mode transition when the database is already in WAL mode
• retry the idempotent pragma sequence once after a 50ms backoff on SQLITE_BUSY
• add memory regression coverage for concurrent fresh opens, already-WAL and readonly opens, and retry behavior

Validation

• npm run build
• npm test
• npm run lint
• npm run test:e2e
• targeted TDD regression proof: 5 passing with fix, 3 failing without fix, 5 passing after restore

Notes

• no schema/data migration or pragma policy changes
• full memory coverage tests pass, but the command retains the existing package-wide branch threshold failure (67.54% vs 75%); connection.ts reports 95.34% statements and 88.46% branches

Risks

• low: synchronous connection bootstrap may add one bounded 50ms delay only after SQLITE_BUSY