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

.changeset/multi-project-migration.md

@@ -1,5 +1,6 @@
 ---
 "@fusion/core": minor
+"@fusion/engine": minor
 "@gsxdsm/fusion": minor
 ---