fix(cli): apply focus to tmux pane for external sessions

[sei40kr]

Description

cli.toggle({ focus = true }) and cli.focus() did not focus the tmux pane when using external mux sessions (cli.mux.enabled = true with create = "split" / "window").

Root cause

sidekick.cli was built around state.terminal (nvim's embedded terminal). When external mux sessions were added, the UI layer (M.toggle / M.focus) still keyed off state.terminal, and Session had no focus() method — so there was literally no path from the UI to tmux select-pane. Terminal focus kept working only because nvim_set_current_win is a synchronous API the UI callback could call directly via a captured opts.focus, bypassing the missing propagation.

Changes

• Add Session:focus() hook on the backend base class (empty stub for backends that don't support it)
• Implement focus() on the tmux backend via tmux select-window + tmux select-pane — select-window first so create = "window" actually switches the client to the pane's window (a no-op for create = "split" where the pane already lives in the current window)
• Unify the UI layer through state.session:focus(). Terminal is already registered as a Session backend (state.terminal === state.session for terminal-backed sessions), so the same call works for both paths:
  • terminal-backed session → Terminal:focus() (existing: show() + nvim_set_current_win)
  • external tmux session → Tmux:focus() (new: tmux select-window + tmux select-pane)
• M.attach now fires focus on opts.show and opts.focus ~= false for both backends, fixing the silent default-false asymmetry external previously had against terminal
• M.toggle callback handles both backends via state.session:focus(), keeping the existing is_open() gate so toggling terminal visibility off doesn't immediately re-open it
• M.focus callback routes to state.terminal:blur() when terminal is focused (existing behavior), otherwise state.session:focus() for both backends

Behavior matrix after the fix

Call	terminal	external tmux
toggle()	toggle visibility + focus	focus pane
toggle({ focus = true })	toggle visibility + focus	focus pane
toggle({ focus = false })	toggle visibility only	no-op
send("x")	show + focus, then send	focus pane, then send
send("x", { focus = false })	show, then send	send only
focus()	toggle focus (blur if focused)	focus pane
hide()	hide window	no-op (filter excludes external)
close()	close terminal	detach tracking (pane stays)

Related Issue(s)

• Fixes bug: focus is not appplied with tmux pane #179

Screenshots

N/A — behavioral fix with no UI change.

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