docs: create onboarding summary

.planning/onboarding/SUMMARY.md

@@ -0,0 +1,19 @@
+# Onboarding Summary
+
+## Project State
+- PROJECT.md: present
+- REQUIREMENTS.md: present
+- ROADMAP.md: present
+- STATE.md: present
+
+## Codebase Context
+- Brownfield repo: yes
+- Map readiness: complete
+- Codebase map: .planning/codebase/ (complete codebase map)
+- Fast map available: yes
+
+## Docs Context
+- Existing ADR/PRD/SPEC/RFC candidates: 11
+
+## Recommended Next Step
+- /gsd-manager