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

Adds `Session:focus()` backend hook with a tmux implementation via
`tmux select-pane`, and unifies the UI layer to dispatch through
`state.session:focus()`. `Terminal` is already a `Session` backend so
`state.terminal === state.session` for terminal-backed sessions — the
same call covers both paths.

Also fixes the default-focus asymmetry in `M.attach`: external focus
now fires on `opts.show and opts.focus ~= false` to match terminal,
so `send()` and `toggle()` (no args) focus external panes the same
way they focus terminals.

Closes #179.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sei40kr

lua/sidekick/cli/init.lua

@@ -100,14 +100,14 @@ end
 function M.toggle(opts)
   opts = filter_opts(opts)
   State.with(function(state, attached)
-    if not state.terminal then
-      return
-    end
-    if not attached then
+    if state.terminal and not attached then
       state.terminal:toggle()
     end
-    if state.terminal:is_open() and opts.focus ~= false then
-      state.terminal:focus()
+    if opts.focus == false then
+      return
+    end
+    if state.external or (state.terminal and state.terminal:is_open()) then
+      state.session:focus()
     end
   end, {
     attach = true,
@@ -121,13 +121,10 @@ end
 function M.focus(opts)
   opts = filter_opts(opts)
   State.with(function(state)
-    if not state.terminal then
-      return
-    end
-    if state.terminal:is_focused() then
+    if state.terminal and state.terminal:is_focused() then
       state.terminal:blur()
     else
-      state.terminal:focus()
+      state.session:focus()
     end
   end, {
     attach = true,

lua/sidekick/cli/session/init.lua

@@ -52,6 +52,9 @@ function B:attach() end
 --- Detach from an existing session
 function B:detach() end
 
+--- Focus an external session (tmux/zellij pane)
+function B:focus() end
+
 --- Start a new session
 --- If the backend returns a Cmd, a new terminal session will be spawned
 ---@return sidekick.cli.terminal.Cmd?

lua/sidekick/cli/session/tmux.lua

@@ -195,4 +195,13 @@ function M:dump()
   return ret
 end
 
+---Focus the tmux pane
+function M:focus()
+  local pane_id = self:pane_id()
+  if not pane_id then
+    return
+  end
+  Util.exec({ "tmux", "select-pane", "-t", pane_id })
+end
+
 return M

lua/sidekick/cli/state.lua

@@ -187,14 +187,14 @@ function M.attach(state, opts)
 
   state = M.get_state(session) -- update state
   local terminal = state.terminal
-  if terminal then
-    if opts.show then
-      terminal:show()
-      if opts.focus ~= false and terminal:is_running() then
-        terminal:focus()
-      end
-    end
-  elseif attached then
+  if terminal and opts.show then
+    terminal:show()
+  end
+  local visible = state.external or (terminal and terminal:is_running())
+  if opts.show and opts.focus ~= false and visible then
+    session:focus()
+  end
+  if not terminal and attached then
     Util.info("Attached to `" .. state.tool.name .. "`")
   end
   return state, attached