Check that every PR description starts with Fixes #N for an open issue #4173

Every pull request description in the Public API is meant to start with Fixes #ISSUE_NUMBER (rule api.process.pr-tracking-issue), but nothing checks it: the rule is doc-only. Recent PRs show the gap, for example #3776 (the Fixes line is on line 37) and every automated PolicyEngine bundle update, such as #4169, whose description starts with ## Summary.

Add a PR description workflow job, ported from the simulation API's check (PolicyEngine/policyengine-sim-api#862), that runs on every pull request event that can change the description (opened, edited, reopened, synchronize) and fails unless:

• the first line of the description is exactly Fixes #N; and
• issue N exists and is open in this repository (looked up with gh api and the job's read-only token).

Then:

• enforce api.process.pr-tracking-issue with that job, with proof fixtures and tests;
• make the automated bundle-update PRs open or reuse a tracking issue and start their descriptions with Fixes #N (the daily relock PR already does).

Making the check required on master is a separate repository-settings change after this merges.