fix: Prevent GitHub API rate limiting from frequent worktree PR fetching

Fixes #685

This commit addresses the GitHub API rate limit issue caused by excessive worktree PR status fetching.

## Changes

### Server-side PR caching (list.ts)
- Added `GitHubPRCacheEntry` interface and `githubPRCache` Map
- Implemented 2-minute TTL cache for GitHub PR data
- Modified `fetchGitHubPRs()` to check cache before making API calls
- Added `forceRefresh` parameter to bypass cache when explicitly requested
- Cache is properly cleared when force refresh is triggered

### Frontend polling reduction (worktree-panel.tsx)
- Increased worktree polling interval from 5 seconds to 30 seconds
- Reduces polling frequency by 6x while keeping UI reasonably fresh
- Updated comment to reflect new polling strategy

### Type improvements (use-worktrees.ts)
- Fixed `fetchWorktrees` callback signature to accept `silent` option
- Returns proper type for removed worktrees detection

## Impact
- Combined ~12x reduction in GitHub API calls
- 2-minute cache prevents repeated API hits during normal operation
- 30-second polling balances responsiveness with API conservation
- Force refresh option allows users to manually update when needed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/ui/src/components/views/board-view/worktree-panel/hooks/use-worktrees.ts

@@ -95,12 +95,20 @@ export function useWorktrees({
   );
 
   // fetchWorktrees for backward compatibility - now just triggers a refetch
-  const fetchWorktrees = useCallback(async () => {
-    await queryClient.invalidateQueries({
-      queryKey: queryKeys.worktrees.all(projectPath),
-    });
-    return refetch();
-  }, [projectPath, queryClient, refetch]);
+  // The silent option is accepted but not used (React Query handles loading states)
+  // Returns removed worktrees array if any were detected, undefined otherwise
+  const fetchWorktrees = useCallback(
+    async (_options?: {
+      silent?: boolean;
+    }): Promise<Array<{ path: string; branch: string }> | undefined> => {
+      await queryClient.invalidateQueries({
+        queryKey: queryKeys.worktrees.all(projectPath),
+      });
+      const result = await refetch();
+      return result.data?.removedWorktrees;
+    },
+    [projectPath, queryClient, refetch]
+  );
 
   const currentWorktreePath = currentWorktree?.path ?? null;
   const selectedWorktree = currentWorktreePath

apps/ui/src/components/views/board-view/worktree-panel/worktree-panel.tsx

@@ -383,13 +383,13 @@ export function WorktreePanel({
 
   const isMobile = useIsMobile();
 
-  // Periodic interval check (5 seconds) to detect branch changes on disk
-  // Reduced from 1s to 5s to minimize GPU/CPU usage from frequent re-renders
+  // Periodic interval check (30 seconds) to detect branch changes on disk
+  // Reduced polling to lessen repeated worktree list calls while keeping UI reasonably fresh
   const intervalRef = useRef<NodeJS.Timeout | null>(null);
   useEffect(() => {
     intervalRef.current = setInterval(() => {
       fetchWorktrees({ silent: true });
-    }, 5000);
+    }, 30000);
 
     return () => {
       if (intervalRef.current) {