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>

cmd/bd/migrate_test.go

@@ -30,7 +30,7 @@ func TestMigrateCommand(t *testing.T) {
 	t.Run("single old database", func(t *testing.T) {
 		// Create old database
 		oldDBPath := filepath.Join(beadsDir, "vc.db")
-		store, err := sqlite.New(oldDBPath)
+		store, err := sqlite.New(context.Background(), oldDBPath)
 		if err != nil {
 			t.Fatalf("Failed to create old database: %v", err)
 		}
@@ -82,7 +82,7 @@ func TestMigrateCommand(t *testing.T) {
 		}
 
 		// Update version
-		store, err := sqlite.New(dbPath)
+		store, err := sqlite.New(context.Background(), dbPath)
 		if err != nil {
 			t.Fatalf("Failed to open database: %v", err)
 		}
@@ -143,7 +143,7 @@ func TestMigrateRespectsConfigJSON(t *testing.T) {
 
 	// Create old database with custom name
 	oldDBPath := filepath.Join(beadsDir, "beady.db")
-	store, err := sqlite.New(oldDBPath)
+	store, err := sqlite.New(context.Background(), oldDBPath)
 	if err != nil {
 		t.Fatalf("Failed to create database: %v", err)
 	}