docs: REUSE.md introduced — cross-repo reuse map (canonical helpers/patterns/traps/seams; docs-only, no version bump)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

CLAUDE.md

@@ -14,6 +14,7 @@ See `controller/README.md` for full architecture and status (update after each s
 See `CHANGELOG.md` for recent work (update after each session — see "Working with CHANGELOG.md" below).
 See `CONTEXT.md` for current project state, decisions and roadmap (update after each session).
 See `TASK.md` for the current task to implement (if it exists).
+See `REUSE.md` before writing new code.
 
 Claude in Chrome extension is available — can be used to test web UI on demo-felhom.eu or verify dashboard deployments in browser.
 
@@ -334,4 +335,5 @@ Before ending a session, always:
 3. **Update CHANGELOG.md** with what was done
 4. **Update CONTEXT.md** with decisions made, update architectural state and what's next
 5. **Update controller/README.md** if architecture or features changed
-6. **Verify** the deployment is working (check `docker ps` and logs)
+6. **Verify** the deployment is working (check `docker ps` and logs)
+7. **Update REUSE.md** if you added/changed/deprecated a shared helper or pattern (same commit)