fix(onboard): enforce complete map gate in fast mode and add skip-ingest rerun

Remove the projectExists guard so fast mode with a partial map still routes
to complete-map-before-new-project even after project planning exists.

Add the missing onboard rerun instruction to the skip-mapping docs-ingest
handoff so the onboarding loop can continue after ingest.

gsd-core/workflows/onboard.md

@@ -98,6 +98,8 @@ Skipping codebase mapping may give downstream steps weaker context, but existing
 Run from worktree root {ONBOARDING_ROOT}:
 
 {handoff_commands.ingest_docs}
+
+Then rerun {handoff_commands.onboard} from the same worktree root.
 ```
 
 Exit.