refactor: rename last_import_hash to jsonl_content_hash (bd-39o)

The metadata key 'last_import_hash' was misleading because it's updated on
both import AND export. Renamed to 'jsonl_content_hash' which more accurately
describes its purpose - tracking the content hash of the JSONL file.

Added migration support: read operations try new key first, then fall back
to old key for backwards compatibility with existing databases.

Files modified:
- cmd/bd/integrity.go: Update key name with migration support
- cmd/bd/import.go: Update key name
- cmd/bd/sync.go: Update key name
- cmd/bd/autoflush.go: Update key name with migration support
- cmd/bd/daemon_sync.go: Update key name
- cmd/bd/daemon_event_loop.go: Update key name with migration support
- internal/autoimport/autoimport.go: Update key name with migration support
- Updated all related tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/integrity.go

@@ -106,9 +106,13 @@ func hasJSONLChanged(ctx context.Context, store storage.Storage, jsonlPath strin
 	}
 
 	// Build metadata keys with optional suffix for per-repo tracking (bd-ar2.10, bd-ar2.11)
-	hashKey := "last_import_hash"
+	// Renamed from last_import_hash to jsonl_content_hash (bd-39o) - more accurate name
+	// since this hash is updated on both import AND export
+	hashKey := "jsonl_content_hash"
+	oldHashKey := "last_import_hash" // Migration: check old key if new key missing
 	if keySuffix != "" {
 		hashKey += ":" + keySuffix
+		oldHashKey += ":" + keySuffix
 	}
 
 	// Always compute content hash (bd-v0y fix)
@@ -121,12 +125,16 @@ func hasJSONLChanged(ctx context.Context, store storage.Storage, jsonlPath strin
 		return false
 	}
 
-	// Get last import hash from metadata
+	// Get content hash from metadata (try new key first, fall back to old for migration)
 	lastHash, err := store.GetMetadata(ctx, hashKey)
-	if err != nil {
-		// No previous import hash - this is the first run or metadata is missing
-		// Assume changed to trigger import
-		return true
+	if err != nil || lastHash == "" {
+		// Try old key for migration (bd-39o)
+		lastHash, err = store.GetMetadata(ctx, oldHashKey)
+		if err != nil || lastHash == "" {
+			// No previous hash - this is the first run or metadata is missing
+			// Assume changed to trigger import
+			return true
+		}
 	}
 
 	// Compare hashes