Point CONTRIBUTING.md at the shared PolicyEngine guide

.github/CONTRIBUTING.md

@@ -1,14 +1,40 @@
-## Updating the versioning
-
-Please add to `changelog_entry.yaml` an entry in the format:
-
-```yaml
-- bump: minor
-  changes:
-    added:
-    - New feature.
-    fixed:
-    - Bug fix.
-    changed:
-    - Change.
+# Contributing to policyengine.py
+
+See the [shared PolicyEngine contribution guide](https://github.com/PolicyEngine/.github/blob/main/CONTRIBUTING.md) for cross-repo conventions (towncrier changelog fragments, `uv run`, PR description format, anti-patterns). This file covers policyengine.py specifics.
+
+## Commands
+
+```bash
+make install                         # install deps (uv)
+make format                          # format (required)
+ruff check .                         # lint
+uv run mypy src/policyengine         # type check
+make test                            # test suite
+make docs                            # build documentation
+
+uv run pytest tests/test_household_impact.py::TestUKHouseholdImpact -v
 ```
+
+Python 3.13+. Default branch: `main`. Tests that download representative datasets need `HUGGING_FACE_TOKEN` set.
+
+## What lives here
+
+policyengine.py is the user-facing analysis package. It wraps `policyengine-uk` and `policyengine-us` with a common `Simulation` object, dataset loaders, and result models (poverty, inequality, decile impacts, programme statistics).
+
+- `src/policyengine/core/` — the shared simulation, dataset, and policy model
+- `src/policyengine/tax_benefit_models/uk/` — UK-specific loaders and analysis
+- `src/policyengine/tax_benefit_models/us/` — US-specific loaders and analysis
+- `src/policyengine/outputs/` — decile/inequality/poverty calculators
+- `src/policyengine/utils/` — parametric-reform helpers, entity utilities
+
+Country pins live in `pyproject.toml` under the `[uk]` / `[us]` / `[dev]` extras. Bumping a pin is a patch-level change most of the time; include the motivation in the PR body.
+
+## Repo-specific anti-patterns
+
+- **Don't** bypass the country-model APIs with direct `policyengine-core` calls from user-facing code. The wrapper exists so analyses survive core API changes.
+- **Don't** add new public classes without Pydantic models for input/output. JSON round-trip is a documented property of the public surface.
+- **Don't** cache arbitrary Python objects — the `core.Simulation` output must be serialisable.
+
+## Examples and notebooks
+
+`examples/` holds runnable scripts that double as documentation. When changing a public API, update or add an example.

changelog.d/docs-towncrier-contributing.changed.md

@@ -0,0 +1 @@
+Point CONTRIBUTING.md at the shared PolicyEngine contribution guide (https://github.com/PolicyEngine/.github) and trim the per-repo file to commands, repo-specific conventions, and anti-patterns. Removes the stale `changelog_entry.yaml` / `make changelog` instructions.