feat(tui): replace custom ratatui TUI with tmux's native UI

[jakemassoth]

Implements the spike recommendation (PR #23, branch spike-tui — the decision was already made): drop the custom ratatui/vt100 TUI and adopt tmux's native UI. Line cooks move from one tmux session per ticket to one shared yeschef session with one window per ticket, and yeschef tui collapses to tmux attach.

What changed

Session/window model — the windows are now real. The whole brigade lives in one yeschef tmux session: the head chef at window 0 and one real tmux window per cook, addressed as yeschef:<window>. That's what lets tmux attach render every cook as a native tab.

The tmux status line is the TUI. tmux.conf ships a window-status-format that renders each tab from a per-window @status user option — a glyph + colour, re-rendered live by tmux with no polling and no custom render code:
CHEF ★ magenta · IN_PROGRESS ● yellow · DONE ✓ green · BLOCKED ■ red · NEW ○ grey.
A new backend method set_window_status pushes @status on every ticket … status-set, right next to the existing SQLite write, so an attached tab bar recolours the instant a cook reports. The window name stays the stable ticket id (the send/peek/kill target); status decoration lives only in @status, so they never collide.

The 3 requirements the TUI must satisfy, all now stock tmux:

1. See / switch cooks — the status-line tab bar; prefix+n/p/<n>, prefix+w tree.
2. Back to the head chef — window 0, always prefix+0 (plus a prefix+c alias).
3. At-a-glance status — colour + glyph per tab, driven by @status.

Detach is stock prefix+d.

Backend delta (TmuxBackend)

Method	Before (session-per-ticket)	After (windows-in-session)
ensure_session	no-op	new-session -d -s yeschef -n headchef … (head chef = window 0, idempotent)
new_window	new-session -d -s yeschef-<w>	new-window -t yeschef: -n <w>
send_keys/capture_pane	-t yeschef-<w>	-t yeschef:<w>
list_windows	list sessions, strip prefix	list-windows -t yeschef (real active/dead)
kill_window	kill-session	kill-window -t yeschef:<w>
attach	attach -t yeschef-<w>	attach -t yeschef (select window if given)
set_window_status	—	new: set-window-option -t yeschef:<w> @status <STATUS>

Deleted

• src/commands/tui.rs — the entire custom TUI (ratatui event loop, vt100 render_pane, PseudoTerminal, head-chef pinning, its unit tests). run_tui is now ~3 lines in orchestrate.rs.
• Deps ratatui, vt100, tui-term (and transitive crossterm); Cargo.lock updated.
• Backend methods that existed only for the custom pane: capture_pane_styled, capture_raw_styled, the shared capture_styled helper, and the bare head-chef-session machinery (ensure_raw_session, attach_raw, names::headchef_session).
• docs/tui-v2-demo.tape (custom-TUI demo); docs/tui-demo.tape rewritten for the native UI.

Net: −~960 lines + 3 crates, +~40 lines of tmux config + one backend method.

How I verified

• cargo fmt --check, cargo clippy -D warnings -D clippy::pedantic, cargo test --bin yeschef (52 unit tests), the --ignored e2e suite (17 tests, rewritten to the windows-in-session model on a throwaway socket), and nix flake check — all green.
• Headless end-to-end run on a throwaway YESCHEF_TMUX_SOCKET: spawned 3 fake cooks, set distinct statuses, and confirmed one yeschef session with head chef window 0 (@status=CHEF) + 3 cook windows carrying the right @status.
• Recorded docs/tui-demo.gif (VHS, embedded above): the coloured tab bar, switching cooks (prefix+n), jumping to the head chef (prefix+0), a live status flip (db-migrate grey ○ → red ■ while attached), and a clean prefix+d detach.

Notes

• Less isolation between cooks (one session, shared server lifecycle) — each cook is still its own window/pane/process and kill-window is per-cook. Confirmed acceptable in the spike.
• Tab bar overflows past ~6–8 cooks (tmux shows </>); prefix+w is the escape hatch — noted in AGENTS.md.
• Based on the latest origin/main (includes the e2e socket-isolation fix, PR fix(tmux): isolate e2e tests on a throwaway socket, never the live yeschef server #24); no rebase needed.