Fix mux tab close after detach/reattach (#407)

[jvanderen1]

Summary

• tmux -CC: optimistic kill + tombstone so ✕ after detach→reattach does not leave empty self-healing trees
• Window-tab close always uses the live gateway requestKillWindow (does not require a live pane in the tab tree)

Depends on / merge after: #430 (this branch now includes current #430, including the herdr control-mode resume commit and the main merge). Review the unique commit for the close-after-reattach change — two files: TmuxController.swift, MainView+TabManagement.swift.

Part of #407. Stack: PR 2 of 3. Still required after #430: that PR does not include the tombstone / requestKillWindow path, so reattached background tmux windows still no-op or self-heal on ✕.

#432 is optional and is not needed to land this.

Screencast

Same DietPi recording as #430 — no separate take. Watch the close/kill beats here; detach/resume is #430.

Different.Multiplexers-gh.mp4

This PR (after #430 detach → reconnect)

1. DietPi tmux -CC, at least 2 windows.
2. Detach Session → reconnect (window tabs return) — Add multiplexer detach/resume (#407) #430.
3. ✕ a background window — tab is gone for good (no ghost / self-heal).

Last herdr tab ✕ ending the session may appear later in the same recording; that is local follow-up, not this diff.

Test plan

• Close reattached tmux window with ✕ (no empty tree / self-heal)
• Detach Session still leaves the tmux session running (Add multiplexer detach/resume (#407) #430)
• Background window ✕ after reattach does not require a live pane in the tab tree