Design: the code browser's explorer column uses the design system, and its ex-* classes leave §5d #3636

For: the design audit (#2678): one page part at a time onto §5's components, so docs/DESIGN.md §5d only shrinks.

Done:

• [ ] The explorer column beside a repository's code is drawn from §5's components (the src/web/ui.ts helpers and §5 classes) in place of .code-main .code-split .ex-empty .ex-find .ex-head .ex-list .ex-name .ex-ref .ex-twist .explorer-toggle (the 'code browser: explorer beside content' bullet), which src/web/style/content.ts defines today.
• [ ] Those rules are gone and their bullet leaves docs/DESIGN.md §5d. bun scripts/design-lint.ts passes.
• [ ] The pull carries before/after screenshots in light and dark, at desktop and 390 px wide. Nothing a person reads or taps is lost, and keyboard use is unchanged.
• [ ] The part's tests pass, and a test finds none of the old class names in its HTML.

Files: src/web/style/content.ts, docs/DESIGN.md, the markup that uses the classes (git grep each), a test. Risk: a class borrowed elsewhere. git grep each name first, and keep any that is used elsewhere, said in the pull.

Parked by the crew: the check failed in the place it ran, not in the change, and failed the same way when it was run again (pull #3814 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Steward (trevato): pull #3814 was parked by an infrastructure fault, not this work - the egress cap from #3719 stopped every check's package install (02:20-03:12Z). The forge is rolled back and the cap is fixed (c7ea1935). Reopened and re-checked; this strike does not count.