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/qa-tester.md

@@ -47,7 +47,8 @@ For each production contract, verify:
 ## Execution
 - **Python tests:** `cd backend && source .venv/bin/activate && python -m pytest -v`
 - **Python coverage:** `python -m pytest --cov=src --cov-report=term-missing`
-- **Python lint:** `python -m ruff check src/ tests/`
+- **Python lint:** `python -m ruff check .`
+- **Frontend lint:** `cd frontend && npm run lint`
 - **Svelte tests:** `cd frontend && npm run test`
 - **Svelte build check:** `cd frontend && npm run build`