Use fzf to pick the agent in agent-cmd.sh

Move agent discovery out of `agent.sh` so the picker runs on every new
pane, including manual splits. Set the agent session's default-command
to agent-cmd.sh and re-apply session options on every invocation so
changes propagate to live sessions.

agent-cmd.sh

@@ -1,8 +1,28 @@
 #!/usr/bin/env bash
-# Wrapper that runs an agent command, then detaches if all panes in window are dead
+# Detect available agents, prompt with fzf, run the selected one.
+# Detaches the client if all panes in the window are dead afterwards.
 
-# Run the agent command (don't use set -e, agent may exit non-zero)
-"$@" || true
+agents=()
+command -v claude &>/dev/null && agents+=(claude)
+command -v opencode &>/dev/null && agents+=(opencode)
+command -v gemini &>/dev/null && agents+=(gemini)
+command -v codex &>/dev/null && agents+=(gemini)
+
+if [ ${#agents[@]} -eq 0 ]; then
+  echo "No agent commands found (claude, opencode, gemini)"
+  exit 1
+fi
+
+agent=$(printf '%s\n' "${agents[@]}" | fzf \
+  --prompt='agent> ' \
+  --reverse \
+  --border=rounded \
+  --margin=25%,30% \
+  --padding=1)
+
+if [ -n "$agent" ]; then
+  "$agent" || true
+fi
 
 # Brief delay to let tmux update pane status
 sleep 0.1