retire the terminal wall: mux is strictly one target, no picker; mux web stays

open   by a73x

Decision 2026-09-05: the native client is the multi-session surface; the terminal client keeps the single-session job and loses the wall.

`mux [TARGET]` becomes strictly one target per invocation: no picker, no tiles, no rails, no split or resize chords, no layout tree or layout file, no host grade, no focus, no `relocateReports`. It keeps the pump and redial, prediction, the askpass popup, `-A`, mouse passthrough, drag select, scroll mode, the end two-step on its own connection, the ssh handoff to QUIC and the exit code a piped `mux` returns.

`mux web` stays and keeps serving the layout file, so `client.layout` and the layout file's owner move to the hub rather than disappearing (`webhub.readLeaves`, `appendLeaf`, `roomForLeaf`). `wall_host.planHostDiff` and `webhub.applyList` collapse to the hub's copy.

Dies with the wall: `wallview.zig` (most), `wall_layout.zig`, `wall_picker.zig`, `wall_host.zig` (all but the poller the hub keeps), the wall parts of `interact.zig`, the six `wall_test_*.zig`, e2e groups 07_wallcli, 09_hosts, 12_panes, 13_birth and the tile legs of 06_web, and the wall invariants in CLAUDE.md. Backlog that closes with it: 32d168d1, the seed half of 55ca6e6d, 2fc534d6, 05fc3090, 7b04a2ac, 68d4700e, most of e3548ab1.

Gated on the GUI gap tickets: d61fdc4c macOS muxg, 8b16e26b mouse/wheel/selection, 50ca9ba5 agent forwarding, f3cf5785 askpass, fb4a0ee4 prediction, 0b725c82 end-session, 08ff372f release bundling. Until all land the wall is FROZEN: bug fixes only, no new wall features.