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/daemon_test.go

@@ -495,7 +495,7 @@ func TestDaemonServerStartFailureSocketExists(t *testing.T) {
 	socketPath := filepath.Join(tmpDir, "test.sock")
 	testDBPath := filepath.Join(tmpDir, "test.db")
 
-	testStore1, err := sqlite.New(testDBPath)
+	testStore1, err := sqlite.New(context.Background(), testDBPath)
 	if err != nil {
 		t.Fatalf("Failed to create test database: %v", err)
 	}
@@ -526,7 +526,7 @@ func TestDaemonServerStartFailureSocketExists(t *testing.T) {
 		t.Fatal("Socket should exist for first server")
 	}
 
-	testStore2, err := sqlite.New(filepath.Join(tmpDir, "test2.db"))
+	testStore2, err := sqlite.New(context.Background(), filepath.Join(tmpDir, "test2.db"))
 	if err != nil {
 		t.Fatalf("Failed to create second test database: %v", err)
 	}