refactor: remove legacy kb compatibility

Drops the .kb/kb.db migration path, legacy backup filename handling, and
backward-compat test suites. Renames internal kbDir identifiers to
fusionDir and hasKbProject/isValidKbProject to their fusion equivalents.

- Remove needsCentralMigration, autoMigrateToCentral, and the
  "needs-migration" FirstRunState; checkAndMigrate and KB_SKIP_MIGRATION
  env var are gone
- Remove LEGACY_BACKUP_DIR and canonicalizeBackupDir; listBackups no
  longer matches kb-* filenames
- Delete backward-compat.test.ts and store-backward-compat.test.ts;
  update remaining tests to new 3-state first-run model

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/chat-store.ts

@@ -44,7 +44,7 @@ export interface ChatStoreEvents {
 
 export class ChatStore extends EventEmitter<ChatStoreEvents> {
   constructor(
-    private kbDir: string,
+    private fusionDir: string,
     private db: Database,
   ) {
     super();