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/routine-store.ts

@@ -51,8 +51,8 @@ export class RoutineStore extends EventEmitter<RoutineStoreEvents> {
    */
   private get db(): Database {
     if (!this._db) {
-      const kbDir = `${this.rootDir}/.fusion`;
-      this._db = new Database(kbDir);
+      const fusionDir = `${this.rootDir}/.fusion`;
+      this._db = new Database(fusionDir);
       this._db.init();
     }
     return this._db;