fix(tmux): pick per-instance ports and restore pane cleanup

.config/fish/functions/c.fish

@@ -1,7 +1,9 @@
 function c --wraps=opencode --description 'opencode (auto-starts tmux for visual subagent panes)'
+    set -l port (python -c 'import socket; s=socket.socket(); s.bind(("127.0.0.1", 0)); print(s.getsockname()[1]); s.close()')
+
     if not set -q TMUX
-        tmux new-session opencode $argv
+        tmux new-session opencode --port $port $argv
     else
-        opencode $argv
+        opencode --port $port $argv
     end
 end