fix(lint): apply QA-found gaps in agent instruction files

- python-coder.md: ruff check src/ tests/ → ruff check .
- speckit.test.md: add frontend npm run lint
- speckit.plan.md: ruff check . + frontend lint
- speckit.tasks.md: ruff check . + frontend lint
- fullstack-coder.md: move npm run lint to frontend block

.opencode/agents/fullstack-coder.md

@@ -63,10 +63,10 @@ You own:
 cd backend && source .venv/bin/activate
 python -m pytest -v
 python -m ruff check .
-npm run lint  # from frontend/
 
 # Frontend
 cd frontend
+npm run lint
 npm run test
 npm run build