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/epic.go

@@ -1,6 +1,5 @@
 package main
 import (
-	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -38,7 +37,7 @@ var epicStatusCmd = &cobra.Command{
 				os.Exit(1)
 			}
 		} else {
-			ctx := context.Background()
+			ctx := rootCtx
 			epics, err = store.GetEpicsEligibleForClosure(ctx)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error getting epic status: %v\n", err)
@@ -120,7 +119,7 @@ var closeEligibleEpicsCmd = &cobra.Command{
 				os.Exit(1)
 			}
 		} else {
-			ctx := context.Background()
+			ctx := rootCtx
 			epics, err := store.GetEpicsEligibleForClosure(ctx)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error getting eligible epics: %v\n", err)
@@ -175,7 +174,7 @@ var closeEligibleEpicsCmd = &cobra.Command{
 					continue
 				}
 			} else {
-				ctx := context.Background()
+				ctx := rootCtx
 				err := store.CloseIssue(ctx, epicStatus.Epic.ID, "All children completed", "system")
 				if err != nil {
 					fmt.Fprintf(os.Stderr, "Error closing %s: %v\n", epicStatus.Epic.ID, err)