fix: v0.10.3 no tmux right-click menus; close split panes

Right-clicking a pane opened tmux's menu, whose Split entries left a stray
pane next to Claude (and whose Kill entries could end it). Unbind the
right-click menus; add --unsplit and a palette entry that closes extra
panes and keeps Claude's. Closes #52

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHRLCCdXYm41Qx5kNhehaP

tests/attach.bats

@@ -1181,3 +1181,17 @@ JSONL
     [ "$(posts)" -eq 1 ]
     [ "$(cat "$DEVDESK_STATE_DIR/usage-warned")" = 2000018000 ]
 }
+
+@test "unsplit closes extra panes and keeps Claude's (#52)" {
+    start_server
+    devdesk new alpha
+    tmux -S "$DEVDESK_SOCKET" split-window -h -t =alpha:claude
+    tmux -S "$DEVDESK_SOCKET" split-window -v -t =alpha:shell
+    run devdesk attach --unsplit alpha
+    [ "$status" -eq 0 ]
+    [ "$(jq -r .closed <<<"$output")" = 2 ]
+    [ "$(tmux -S "$DEVDESK_SOCKET" list-panes -t =alpha:claude -F '#{pane_start_command}')" = "$REPO/libexec/devdesk-claude-run" ]
+    [ "$(tmux -S "$DEVDESK_SOCKET" list-panes -t =alpha:shell | wc -l)" = 1 ]
+    run devdesk attach --unsplit alpha
+    [ "$(jq -r .closed <<<"$output")" = 0 ]
+}