Align changelog checks with Towncrier

[anth-volk]

Fixes #374

Summary

• Replace the recursive find changelog gate with Towncrier's native check --compare-with origin/main, matching the policyengine-api-v2-alpha pattern.
• Fetch full history in the changelog-check job so Towncrier can compare against origin/main.
• Move the previously merged UK geography changelog text into a Towncrier-valid numeric fragment so the next main versioning run consumes it.
• Add focused tests covering flat fragment bump inference and the nested-fragment failure mode that caused the drift.
• Port the model-neutral AI guidance harness pattern from policyengine-us-data: AGENTS.md, thin Claude/Copilot adapters, canonical skills under docs/engineering/skills/, and a same-repository draft PR workflow.
• Add make lint and make push-pr-branch so the required pre-PR lint/format and canonical branch push steps are executable.

Verification

• make format
• make lint
• .venv/bin/python -m pytest --noconftest tests/test_bump_version.py -q
• env PATH=.venv/bin:$PATH .github/check-changelog.sh origin/main
• .venv/bin/towncrier build --draft --version 0.0.0
• make push-pr-branch