fix: bd migrate respects config.json database name and fixes I/O errors

Fixes #204

Multiple critical bugs in bd migrate:

1. **Respects config.json database name**: migrate now loads config.json
   and uses the configured database name instead of hardcoding beads.db.
   Users with custom database names (e.g., beady.db) will no longer
   have their databases renamed.

2. **Fixes disk I/O error (522)**: Clean up orphaned WAL files before
   reopening database to update schema version. This prevents SQLite
   error 522 (disk I/O error) when WAL files exist from previous
   database sessions.

3. **Creates backup before migration**: First migration now creates
   a timestamped backup (*.backup-pre-migrate-*.db) before renaming
   database, consistent with hash-id migration behavior.

4. **Cleans up orphaned WAL files**: Removes .db-wal and .db-shm
   files after migrating old database to prevent stale files.

Changes:
- Load config.json in migrate command to get target database name
- Use cfg.Database instead of hardcoded beads.CanonicalDatabaseName
- Add loadOrCreateConfig() helper function
- Add cleanupWALFiles() to remove orphaned WAL/SHM files
- Create backup before first migration
- Clean up WAL files before version update and after migration
- Add TestMigrateRespectsConfigJSON test

All migration tests passing.

Amp-Thread-ID: https://ampcode.com/threads/T-e5b9ddd0-621b-418b-bc52-ba9462975c39
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/migrate_test.go

@@ -125,3 +125,53 @@ func TestFormatDBList(t *testing.T) {
 		t.Errorf("Expected version 0.16.0, got %s", result[1]["version"])
 	}
 }
+
+func TestMigrateRespectsConfigJSON(t *testing.T) {
+	// Test that migrate respects custom database name from config.json
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0750); err != nil {
+		t.Fatalf("Failed to create .beads directory: %v", err)
+	}
+
+	// Create config.json with custom database name
+	configPath := filepath.Join(beadsDir, "config.json")
+	configData := `{"database": "beady.db", "version": "0.21.1", "jsonl_export": "beady.jsonl"}`
+	if err := os.WriteFile(configPath, []byte(configData), 0600); err != nil {
+		t.Fatalf("Failed to create config.json: %v", err)
+	}
+
+	// Create old database with custom name
+	oldDBPath := filepath.Join(beadsDir, "beady.db")
+	store, err := sqlite.New(oldDBPath)
+	if err != nil {
+		t.Fatalf("Failed to create database: %v", err)
+	}
+	ctx := context.Background()
+	if err := store.SetMetadata(ctx, "bd_version", "0.21.1"); err != nil {
+		t.Fatalf("Failed to set version: %v", err)
+	}
+	_ = store.Close()
+
+	// Load config
+	cfg, err := loadOrCreateConfig(beadsDir)
+	if err != nil {
+		t.Fatalf("Failed to load config: %v", err)
+	}
+
+	// Verify config respects custom database name
+	if cfg.Database != "beady.db" {
+		t.Errorf("Expected database name 'beady.db', got %s", cfg.Database)
+	}
+
+	expectedPath := filepath.Join(beadsDir, "beady.db")
+	actualPath := cfg.DatabasePath(beadsDir)
+	if actualPath != expectedPath {
+		t.Errorf("Expected path %s, got %s", expectedPath, actualPath)
+	}
+
+	// Verify database exists at custom path
+	if _, err := os.Stat(actualPath); os.IsNotExist(err) {
+		t.Errorf("Database does not exist at custom path: %s", actualPath)
+	}
+}