A commit's head and message draw §5 parts on the commit and report pages, and .commit-head, .commit-body leave the stylesheet #4892

Intent: #4585 rewritten. Its pull #4648 stayed red, and #4723 died on a docs/DESIGN.md conflict, which the issue this waits for removes.

Done:

• [ ] in src/web/pages/commits.ts, the commit-head block draws with §5's card head (.card-head) or the page head, and the commit-body message with §5's .code block (it is preformatted text). Say which in the pull. src/web/pages/reports.ts's commit-body does the same
• [ ] .commit-head and .commit-body leave src/web/style/content.ts and their §5d lines go; #4586 (the same rules) is closed by the same pull
• [ ] a test fetches a commit page and a report page and asserts the message is shown in the new part with no commit-head or commit-body; a look of the commit page is in the pull

Files: commits.ts, reports.ts, content.ts, docs/DESIGN.md and one test.

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.

Closed: already done on main in d106e291 (pull #5181).