Add UnderlyingDB() method for extension database access

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>

internal/storage/sqlite/sqlite.go

@@ -1899,3 +1899,9 @@ func (s *SQLiteStorage) Close() error {
 func (s *SQLiteStorage) Path() string {
 	return s.dbPath
 }
+
+// UnderlyingDB returns the underlying *sql.DB connection
+// This allows extensions (like VC) to create their own tables in the same database
+func (s *SQLiteStorage) UnderlyingDB() *sql.DB {
+	return s.db
+}