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/show.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -22,7 +21,7 @@ var showCmd = &cobra.Command{
 	Args:  cobra.MinimumNArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
 		jsonOutput, _ := cmd.Flags().GetBool("json")
-		ctx := context.Background()
+		ctx := rootCtx
 
 		// Check database freshness before reading (bd-2q6d, bd-c4rq)
 		// Skip check when using daemon (daemon auto-imports on staleness)
@@ -400,7 +399,7 @@ var updateCmd = &cobra.Command{
 			return
 		}
 
-		ctx := context.Background()
+		ctx := rootCtx
 
 		// Resolve partial IDs first
 		var resolvedIDs []string
@@ -532,7 +531,7 @@ Examples:
 	Args: cobra.ExactArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
 		id := args[0]
-		ctx := context.Background()
+		ctx := rootCtx
 
 		// Resolve partial ID if in direct mode
 		if daemonClient == nil {
@@ -724,7 +723,7 @@ var closeCmd = &cobra.Command{
 		}
 		jsonOutput, _ := cmd.Flags().GetBool("json")
 
-		ctx := context.Background()
+		ctx := rootCtx
 
 		// Resolve partial IDs first
 		var resolvedIDs []string