The dead .issue-head and .issue-meta rules leave content.ts, and the tests that name them assert their absence #4890

Intent: #4605 rewritten. Its pull #4644 died on a docs/DESIGN.md conflict, which the issue this waits for removes. No markup draws these classes on main at 61723edd.

Done:

• [ ] the .issue-head and .issue-meta rules (including .issue-head h1 .number) leave src/web/style/content.ts, and their §5d lines go. If .number then has no rule, its line goes too
• [ ] test/commit-page-timing.test.ts, studio-changes, studio-runs, web-apps and web-browse: any assertion that these classes are present becomes one that they are absent; assertions of absence stay
• [ ] no markup change

Files: src/web/style/content.ts, docs/DESIGN.md and those tests only.

Run bun scripts/design-lint.ts and bun test test/design-lint.test.ts before pushing; lower docs/design/baseline.json only where a count fell. In docs/DESIGN.md, delete only your classes' own lines from §5d (one class per line since the issue this waits for), so parallel pulls do not conflict. If a test you did not touch goes red, name it in the pull and do not fix it here.