Finish migration from yaml-changelog to towncrier

[MaxGhenis]

Summary

This repo was partway through the yaml-changelog → towncrier migration already:

• pr.yaml runs towncrier check --compare-with origin/main
• .github/bump_version.py infers the version bump from changelog.d/ fragment types
• pyproject.toml carries [tool.towncrier] config with the five canonical fragment categories

What remained (stale, unused, or actively misleading):

• yaml-changelog>=0.1.7 in pyproject.toml dev-deps — unused
• .github/workflows/reusable_changelog_check.yaml — not wired into any workflow call-graph
• changelog_entry.yaml at the repo root — zero bytes
• changelog.yaml at the repo root — 601 lines, entire contents already compiled into CHANGELOG.md
• .github/CONTRIBUTING.md — described the old yaml-changelog flow and told contributors to edit changelog_entry.yaml

This PR drops all of the above and rewrites .github/CONTRIBUTING.md to point at the new shared PolicyEngine guide (PolicyEngine/.github#3) + a repo-specific section covering commands, test-placement rules, dataset-versioning, and anti-patterns.

After this lands, the repo's changelog tooling matches the rest of the org (policyengine-uk-data, policyengine-core, policyengine-uk, policyengine-us, microdf, policyengine.py) on a single towncrier flow.

Diff shape

• 6 files changed, +63 / −649 (the large minus is mostly the retired changelog.yaml).
• New changelog.d/.gitkeep (replacing the empty changelog_entry.yaml) and a single changelog.d/migrate-to-towncrier.changed.md fragment documenting this migration.

Test plan

• pyproject.toml no longer references yaml-changelog
• changelog_entry.yaml and changelog.yaml removed; history preserved in CHANGELOG.md and git log
• reusable_changelog_check.yaml removed (was already unused)
• changelog.d/.gitkeep exists so the towncrier directory survives
• CI check-changelog (towncrier check) passes against this PR
• Versioning workflow on merge runs towncrier build cleanly against the new fragment