chore: configure ruff + eslint linters for agentic workflow

- Replace pylint with ruff (backend/ruff.toml)
  - line-length=300, max-complexity=10, per-file-ignores for tests
  - Exclude D (docstrings replaced by contracts) and ANN
- Add eslint flat config for frontend (eslint-plugin-svelte)
  - Allow console.info/warn/error (belief-state protocol)
- Remove .pylintrc (references non-existent plugin)
- Update agent docs: ruff check . + npm run lint
- Add ruff>=0.11.0 to backend/requirements.txt
- Add eslint + plugins to frontend devDependencies

.opencode/agents/fullstack-coder.md

@@ -62,7 +62,8 @@ You own:
 # Backend
 cd backend && source .venv/bin/activate
 python -m pytest -v
-python -m ruff check src/ tests/
+python -m ruff check .
+npm run lint  # from frontend/
 
 # Frontend
 cd frontend