Fix bugs 2 and 3: agent panel expand and stale worktree references

Bug 2: Expand triangle now works when no agents are started - shows
"No agents started" message. AgentPanel moved to top of panels.

Bug 3: Run `git worktree prune` before `git worktree add` to clean
stale references from externally-deleted worktree directories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/AgentPanel.tsx

@@ -471,17 +471,23 @@ export function AgentPanel({ stories }: AgentPanelProps) {
 								>
 									<button
 										type="button"
-										onClick={() =>
+										onClick={() => {
+											const isExpanded =
+												expandedKey?.startsWith(`${story.story_id}:`) ||
+												expandedKey === story.story_id;
 											setExpandedKey(
-												expandedKey?.startsWith(`${story.story_id}:`)
+												isExpanded
 													? null
 													: (storyAgentEntries[0]?.[0] ?? story.story_id),
-											)
-										}
+											);
+										}}
 										onKeyDown={(e) => {
 											if (e.key === "Enter" || e.key === " ") {
+												const isExpanded =
+													expandedKey?.startsWith(`${story.story_id}:`) ||
+													expandedKey === story.story_id;
 												setExpandedKey(
-													expandedKey?.startsWith(`${story.story_id}:`)
+													isExpanded
 														? null
 														: (storyAgentEntries[0]?.[0] ?? story.story_id),
 												);
@@ -494,9 +500,11 @@ export function AgentPanel({ stories }: AgentPanelProps) {
 											cursor: "pointer",
 											fontSize: "0.8em",
 											padding: "0 4px",
-											transform: expandedKey?.startsWith(`${story.story_id}:`)
-												? "rotate(90deg)"
-												: "rotate(0deg)",
+											transform:
+												expandedKey?.startsWith(`${story.story_id}:`) ||
+												expandedKey === story.story_id
+													? "rotate(90deg)"
+													: "rotate(0deg)",
 											transition: "transform 0.15s",
 										}}
 									>
@@ -643,6 +651,21 @@ export function AgentPanel({ stories }: AgentPanelProps) {
 									)}
 								</div>
 
+								{/* Empty state when expanded with no agents */}
+								{expandedKey === story.story_id && storyAgentEntries.length === 0 && (
+									<div
+										style={{
+											borderTop: "1px solid #2a2a2a",
+											padding: "12px",
+											fontSize: "0.8em",
+											color: "#555",
+											textAlign: "center",
+										}}
+									>
+										No agents started. Use the Run button to start an agent.
+									</div>
+								)}
+
 								{/* Expanded detail per agent */}
 								{storyAgentEntries.map(([key, a]) => {
 									if (expandedKey !== key) return null;