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/prefix_validation_test.go

@@ -17,13 +17,15 @@ func TestPrefixValidation(t *testing.T) {
 	defer os.RemoveAll(tmpDir)
 
 	dbPath := filepath.Join(tmpDir, "test.db")
-	store, err := New(dbPath)
+	ctx := context.Background()
+
+	store, err := New(ctx, dbPath)
 	if err != nil {
 		t.Fatalf("failed to create storage: %v", err)
 	}
 	defer store.Close()
 
-	ctx := context.Background()
+	ctx = context.Background()
 
 	// Set prefix to "test"
 	if err := store.SetConfig(ctx, "issue_prefix", "test"); err != nil {
@@ -93,13 +95,15 @@ func TestPrefixValidationBatch(t *testing.T) {
 	defer os.RemoveAll(tmpDir)
 
 	dbPath := filepath.Join(tmpDir, "test.db")
-	store, err := New(dbPath)
+	ctx := context.Background()
+
+	store, err := New(ctx, dbPath)
 	if err != nil {
 		t.Fatalf("failed to create storage: %v", err)
 	}
 	defer store.Close()
 
-	ctx := context.Background()
+	ctx = context.Background()
 
 	// Set prefix to "batch"
 	if err := store.SetConfig(ctx, "issue_prefix", "batch"); err != nil {