Context propagation with graceful cancellation (bd-rtp, bd-yb8, bd-2o2)

Complete implementation of signal-aware context propagation for graceful
cancellation across all commands and storage operations.

Key changes:

1. Signal-aware contexts (bd-rtp):
   - Added rootCtx/rootCancel in main.go using signal.NotifyContext()
   - Set up in PersistentPreRun, cancelled in PersistentPostRun
   - Daemon uses same pattern in runDaemonLoop()
   - Handles SIGINT/SIGTERM for graceful shutdown

2. Context propagation (bd-yb8):
   - All commands now use rootCtx instead of context.Background()
   - sqlite.New() receives context for cancellable operations
   - Database operations respect context cancellation
   - Storage layer propagates context through all queries

3. Cancellation tests (bd-2o2):
   - Added import_cancellation_test.go with comprehensive tests
   - Added export cancellation test in export_test.go
   - Tests verify database integrity after cancellation
   - All cancellation tests passing

Fixes applied during review:
   - Fixed rootCtx lifecycle (removed premature defer from PersistentPreRun)
   - Fixed test context contamination (reset rootCtx in test cleanup)
   - Fixed export tests missing context setup

Impact:
   - Pressing Ctrl+C during import/export now cancels gracefully
   - No database corruption or hanging transactions
   - Clean shutdown of all operations

Tested:
   - go build ./cmd/bd ✓
   - go test ./cmd/bd -run TestImportCancellation ✓
   - go test ./cmd/bd -run TestExportCommand ✓
   - Manual Ctrl+C testing verified

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/sqlite.go

@@ -76,7 +76,7 @@ func init() {
 }
 
 // New creates a new SQLite storage backend
-func New(path string) (*SQLiteStorage, error) {
+func New(ctx context.Context, path string) (*SQLiteStorage, error) {
 	// Build connection string with proper URI syntax
 	// For :memory: databases, use shared cache so multiple connections see the same data
 	var connStr string
@@ -180,7 +180,6 @@ func New(path string) (*SQLiteStorage, error) {
 	// Hydrate from multi-repo config if configured (bd-307)
 	// Skip for in-memory databases (used in tests)
 	if path != ":memory:" {
-		ctx := context.Background()
 		_, err := storage.HydrateFromMultiRepo(ctx)
 		if err != nil {
 			return nil, fmt.Errorf("failed to hydrate from multi-repo: %w", err)