docs: add ruff linting/formatting conventions for Python repos

CODING-CONVENTIONS.md

@@ -88,6 +88,54 @@ uv lock
 uv run pytest
 ```
 
+### Code Formatting & Linting (Ruff)
+
+All Python code must pass `ruff check` and `ruff format` before merge. Ruff is configured in each repo's `pyproject.toml`:
+
+```toml
+[tool.ruff]
+line-length = 100
+target-version = "py311"
+
+[tool.ruff.lint]
+select = ["E", "F", "W", "I", "UP", "B", "C4", "SIM"]
+ignore = ["E501"]  # Line length handled by formatter
+
+[tool.ruff.format]
+quote-style = "double"
+```
+
+**Required dev dependency:**
+```toml
+[project.optional-dependencies]
+dev = [
+    "pytest>=8.0.0",
+    "pytest-asyncio>=0.23.0",
+    "pytest-cov>=4.0.0",  # For coverage in handler-base
+    "ruff>=0.1.0",
+]
+```
+
+**Local workflow:**
+```bash
+# Check and auto-fix
+uv run ruff check --fix .
+
+# Format code
+uv run ruff format .
+
+# Verify before commit
+uv run ruff check . && uv run ruff format --check .
+```
+
+**CI enforcement:** All repos run ruff in the lint job. Commits that fail linting will not pass CI.
+
+**Kubeflow pipeline variables:** For Kubeflow DSL pipelines, terminal task assignments that appear unused should have `# noqa: F841` comments, as these define the DAG structure:
+```python
+# Step 6: Final step (defines DAG dependency)
+tts_task = synthesize_speech(text=llm_task.output)  # noqa: F841
+```
+
 ### Project Structure
 
 ```python