feat: check the core repos against the standard every week - #118

Summary

Every drift the manual audits found built up slowly, one repo at a time, and was only seen when someone went looking. This adds the check that keeps the now-clean baseline clean.

• conformance/: a checker whose checks are named by requirement ID. It finds the core repos itself (a PyPI project that links back to github.com/modern-python/<repo>, so a PyPI name clash like chat-app is not core), clones each, reads its GitHub metadata and recent PR titles, and evaluates it.
• .github/workflows/conformance.yml: runs it every Monday and on dispatch, writes the summary to the run page, and keeps one conformance-labelled issue per failing repo in this repo (find-or-comment, the same shape as links.yml), closing it once the repo passes. Issues live here because the workflow token cannot write to other repos.

How it reads the standard

• Exemptions come from the standard's table, so a PR adding a row is still what grants one. the core as a whole, except [TS2] is understood, and a PV1 exemption drops the free-threaded entry from the PV2 and MD4 expectations too.
• SHOULD and phase-ins report as warnings: RL6 (PR titles), CI6 floors-matrix drops (allowed only where no wheel exists, which a script cannot see), and TS6 bare pragmas until 2026-11-01, failures after.
• Not checked automatically: JF3, PV3 (permissions), FL3 (glossary content), FL6 (an override is allowed and indistinguishable from drift), RM2 (wording of the opening). The summary says so.
• RL5 compares release.yml across repos ignoring action version pins, so a Dependabot bump in one repo is not drift.

Current result

Run locally against the org: 24 core repos, 0 failing, 5 warnings (three documented floors drops, two old non-conventional PR titles).

Checklist

• uv run pytest: 158 passed, 29 skipped (25 new tests: a fully conforming fixture repo, one failure per sampled requirement, exemptions, the TS6 date switch, RL5 consistency, scope, and parsing the real standard.md and profile)
• mkdocs build --strict passes
• End-to-end run against the org (above)
• The issue-sync step has not run yet; the first scheduled or dispatched run exercises it