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/create.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -168,7 +167,7 @@ var createCmd = &cobra.Command{
 		// In daemon mode, the parent will be sent to the RPC handler
 		// In direct mode, we generate the child ID here
 		if parentID != "" && daemonClient == nil {
-			ctx := context.Background()
+			ctx := rootCtx
 			childID, err := store.GetNextChildID(ctx, parentID)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
@@ -186,7 +185,7 @@ var createCmd = &cobra.Command{
 			}
 
 			// Validate prefix matches database prefix
-			ctx := context.Background()
+			ctx := rootCtx
 
 			// Get database prefix from config
 			var dbPrefix string
@@ -263,7 +262,7 @@ var createCmd = &cobra.Command{
 			ExternalRef:        externalRefPtr,
 		}
 
-		ctx := context.Background()
+		ctx := rootCtx
 		
 		// Check if any dependencies are discovered-from type
 		// If so, inherit source_repo from the parent issue