Implements database platform layer for extensions like VC to create
their own tables in the same SQLite database.
Changes:
- Add UnderlyingDB() *sql.DB to Storage interface
- Implement in SQLiteStorage to expose underlying connection
- Add comprehensive test suite (5 tests, -race clean)
- Tests cover: basic access, extension tables, concurrency,
lifecycle safety, and transaction behavior
This allows VC to host its executor_instances and other tables
alongside beads core tables with proper FK enforcement.
Related issues: bd-57, bd-64, bd-65, bd-66
Amp-Thread-ID: https://ampcode.com/threads/T-a6715beb-fe92-4dee-b931-3c9327124875
Co-authored-by: Amp <amp@ampcode.com>
