#24938 cmux NEXT composer: remove the bottom-left harness label

Leo's composer ask (2026-10-24):

"the biggest one is why did you guys still not remove the bottom left thing? Like, nobody asked for this, literally. Like, I'm pretty sure you guys got confused with, like, the harness on the left type thing, but it was, like, well, yeah, the harness on the left for the drop-down or whatever, you know, the model picker. It's still, like, awkward as hell here. I don't know what you guys are doing, like showing all the machines on here as well. Like, that's not what the intention was."

This PR handles the first half of that ask: the composer footer no longer renders a harness label or its dedicated CSS/component. The model picker remains the harness-selection surface; the existing machine/location selection is untouched. The gallery fixture and browser assertions now verify the footer has only its mode/model controls and that a new chat opens the model picker rail for harness selection.

Gallery before/after (base feat-cmux-next vs this head): composer gallery compare

Validation:

• bun run typecheck (webviews): pass.
• Focused row/footer unit tests: 10 pass.
• Browser suites are wired and skip locally without the fleet browser lane; the local Chromium probe was inconclusive under host memory pressure (exit 143).
• bun run check:browser-lanes and targeted vp fmt --check: pass.

The overlap scan reported neighboring draft PRs #24519 and #24391 on shared composer files; neither changes this footer harness removal.

Takes back #24169: remove the obsolete harness footer label while keeping harness selection in the model picker.

Agent session: big-red

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}