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/python-coder.md

@@ -71,7 +71,7 @@ cd backend && source .venv/bin/activate && python -m pytest -v
 python -m pytest --cov=src --cov-report=term-missing
 
 # Ruff linting
-python -m ruff check src/ tests/
+python -m ruff check .
 
 # Specific test file
 python -m pytest tests/test_auth.py -v