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/Chat.tsx

@@ -703,6 +703,9 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 					margin: "0 auto",
 					width: "100%",
 					padding: "12px 24px 0",
+					flexShrink: 1,
+					overflowY: "auto",
+					minHeight: 0,
 				}}
 			>
 				<div
@@ -712,6 +715,8 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 						gap: "12px",
 					}}
 				>
+					<AgentPanel stories={upcomingStories} />
+
 					<ReviewPanel
 						reviewQueue={reviewQueue}
 						isReviewLoading={isReviewLoading}
@@ -754,8 +759,6 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 						lastRefresh={lastUpcomingRefresh}
 						onRefresh={refreshUpcomingStories}
 					/>
-
-					<AgentPanel stories={upcomingStories} />
 				</div>
 			</div>