feat(KB-504): add migration and first-run experience for multi-project support

- Add MigrationOrchestrator with filesystem scanning and auto-registration

- Add FirstRunExperience with setup wizard state management

- CLI integration with automatic migration hook on first run

- Backward-compatible single-project mode support

- \"--project\" flag support for CLI multi-project targeting

- KB_SKIP_MIGRATION environment variable for recovery

- Graceful fallback when central database unavailable

- New fn init command to initialize kb projects

- Interactive first-run setup wizard API in dashboard

packages/engine/src/index.ts

@@ -14,3 +14,4 @@ export { PrCommentHandler } from "./pr-comment-handler.js";
 export { NtfyNotifier, type NtfyNotifierOptions } from "./notifier.js";
 export { CronRunner, type CronRunnerOptions } from "./cron-runner.js";
 export { StuckTaskDetector, type StuckTaskDetectorOptions, type DisposableSession } from "./stuck-task-detector.js";
+export { ProjectManager } from "./project-manager.js";