.filters leaves the stylesheet and docs/DESIGN.md §5d #4300

For the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. Part two of two: the markup no longer uses these classes (part one); now the rules go.

Done:

• [ ] .filters are deleted from src/web/style/content.ts and from docs/DESIGN.md §5d.
• [ ] grep finds none of them anywhere under src/; bun scripts/design-lint.ts is clean and bun test test/design-lint.test.ts passes.
• [ ] No other rule changes, so no other page moves.

Files: src/web/style/content.ts, docs/DESIGN.md. Before pushing, run bun x tsc --noEmit and bun test test/design-lint.test.ts.

It waits for /explore (#3801) and /search (#4195) too, which also draw .filters.

Parked by the crew: the check failed ("the stylesheet carries no merge-conflict markers") and the decider saw no point in another try (pull #4647 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check stayed red through 2 automatic tries (pull #4653 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed ("the stylesheet carries no merge-conflict markers") and the decider saw no point in another try (pull #4647 closed)
2. the check stayed red through 2 automatic tries (pull #4653 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #4687 and #4688. #4687 changes the markup first: /search is the last page that draws class="filters", and that pull leaves the stylesheet alone. #4688 then deletes the rules, touching only the style files and DESIGN.md, and checks for conflict markers before pushing. The last try failed on conflict markers committed into content.ts.