Add reusable lint and changelog workflows

[MaxGhenis]

Summary

• Adds two reusable GitHub Actions workflows to the org-level .github repo
• lint.yml: Runs uvx ruff format --check . (standardizes the lint step used across 10+ repos)
• changelog.yml: Checks for a changelog fragment in changelog.d/ (identical logic currently copy-pasted across 10+ repos)

Usage

Any PolicyEngine repo can replace its inline lint/changelog jobs with:

jobs:
  Lint:
    uses: PolicyEngine/.github/.github/workflows/lint.yml@main
  check-changelog:
    uses: PolicyEngine/.github/.github/workflows/changelog.yml@main

Survey of current patterns

The following repos all have essentially the same lint and changelog steps, with minor variations in how ruff is installed:

Repo	Has lint?	Has changelog check?
policyengine-core	uvx ruff format --check .	Yes
policyengine-us	uv run ruff format --check .	Yes
policyengine-uk	uvx ruff format --check .	No (should add)
policyengine-canada	pip install ruff && ruff format --check .	Yes
policyengine-il	pip install ruff && ruff format --check .	Yes
policyengine-ng	pip install ruff && ruff format --check .	Yes
policyengine-sg	None	Yes
policyengine-nz	uv run ruff format --check .	Yes
policyengine-us-data	pip install ruff && ruff format --check . (reusable)	Yes
policyengine-uk-data	N/A	Yes
microdf	N/A	Yes
L0	black (outdated)	N/A

Test plan

• Merge this PR
• Create follow-up PRs on 2-3 repos (policyengine-il, policyengine-ng) to use these reusable workflows
• Verify CI passes on those repos

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