feat(KB-620): add multi-project first-run migration flow

- Add first-run migration detection and registration helpers for central project setup
- Wire CLI and dashboard setup routes to auto-migrate existing projects and complete setup
- Harden path resolution, overlap checks, and backward-compat project selection behavior
- Add regression tests for migration, nested project detection, and single-project compatibility

.changeset/multi-project-migration.md

@@ -1,19 +1,13 @@
 ---
 "@fusion/core": minor
-"@fusion/engine": minor
 "@gsxdsm/fusion": minor
 ---
 
 Add migration and first-run experience for multi-project support
 
-- Auto-detect and register existing kb projects on first run
-- MigrationOrchestrator with filesystem scanning and safety checks
-- FirstRunExperience with setup wizard state management
-- Backward-compatible single-project mode (TaskStore.getOrCreateForProject)
-- `--project` flag support for CLI multi-project targeting
-- `KB_SKIP_MIGRATION` environment variable for recovery
-- Graceful fallback when central database unavailable
+- Auto-detect and register existing projects on first run after upgrade
 - New `fn init` command to initialize kb projects
+- Maintain backward compatibility for single-project workflows
 - Interactive first-run setup wizard API in dashboard
 - Idempotent migration — safe to re-run
 - Rollback procedure documented in AGENTS.md