fix: stop repeated GitHub PR fetch warnings for non-GitHub repos

When opening a git repository without a GitHub remote, the server logs
were spammed with warnings every 5 seconds during worktree polling:

  WARN [Worktree] Failed to fetch GitHub PRs: Command failed: gh pr list
  ... no git remotes found

This happened because fetchGitHubPRs() ran `gh pr list` without first
checking if the project has a GitHub remote configured.

Changes:
- Add per-project cache for GitHub remote status with 5-minute TTL
- Check cache before attempting to fetch PRs, skip silently if no remote
- Add forceRefreshGitHub parameter to clear cache on manual refresh
- Pass forceRefreshGitHub when user clicks the refresh worktrees button

This allows users to add a GitHub remote and immediately detect it by
clicking the refresh button, while preventing log spam during normal
polling for projects without GitHub remotes.

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

@@ -39,7 +39,10 @@ export function useWorktrees({
           logger.warn('Worktree API not available');
           return;
         }
-        const result = await api.worktree.listAll(projectPath, true);
+        // Pass forceRefreshGitHub when this is a manual refresh (not silent polling)
+        // This clears the GitHub remote cache so users can re-detect after adding a remote
+        const forceRefreshGitHub = !silent;
+        const result = await api.worktree.listAll(projectPath, true, forceRefreshGitHub);
         if (result.success && result.worktrees) {
           setWorktrees(result.worktrees);
           setWorktreesInStore(projectPath, result.worktrees);