feat: Add 'bd stale' command to show and release orphaned executor claims

- Implements bd stale command to show issues with execution_state where executor is dead/stopped
- Adds --release flag to automatically release orphaned issues
- Adds --threshold flag to customize heartbeat staleness threshold (default: 300s/5min)
- Handles missing executor instances (LEFT JOIN) for cases where executor was deleted
- Adds QueryContext and BeginTx helper methods to SQLiteStorage for advanced queries
- Fixes ExternalRef comparison bug in import_shared.go (pointer vs string)
- Removes unused imports in import.go

Resolves vc-124

internal/storage/sqlite/util.go

@@ -0,0 +1,18 @@
+package sqlite
+
+import (
+	"context"
+	"database/sql"
+)
+
+// QueryContext exposes the underlying database QueryContext method for advanced queries
+// This is used by commands that need direct SQL access (e.g., bd stale)
+func (s *SQLiteStorage) QueryContext(ctx context.Context, query string, args ...interface{}) (*sql.Rows, error) {
+	return s.db.QueryContext(ctx, query, args...)
+}
+
+// BeginTx starts a new database transaction
+// This is used by commands that need to perform multiple operations atomically
+func (s *SQLiteStorage) BeginTx(ctx context.Context) (*sql.Tx, error) {
+	return s.db.BeginTx(ctx, nil)
+}