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/import.go

@@ -3,7 +3,6 @@ package main
 import (
 	"bufio"
 	"bytes"
-	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -51,7 +50,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 			daemonClient = nil
 
 			var err error
-			store, err = sqlite.New(dbPath)
+			store, err = sqlite.New(rootCtx, dbPath)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "Error: failed to open database: %v\n", err)
 				os.Exit(1)
@@ -89,7 +88,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 		}
 
 		// Phase 1: Read and parse all JSONL
-		ctx := context.Background()
+		ctx := rootCtx
 		scanner := bufio.NewScanner(in)
 
 		var allIssues []*types.Issue
@@ -175,7 +174,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 
 		// Check if database needs initialization (prefix not set)
 		// Detect prefix from the imported issues
-		initCtx := context.Background()
+		initCtx := rootCtx
 		configuredPrefix, err2 := store.GetConfig(initCtx, "issue_prefix")
 		if err2 != nil || strings.TrimSpace(configuredPrefix) == "" {
 			// Database exists but not initialized - detect prefix from issues