fix: skip symlinks in copyDirSync to prevent infinite recursion

Adversarial review caught that .claude/skills/gstack may be a symlink
back to the repo root, causing copyDirSync to recurse infinitely
when copying gitignored artifacts into worktrees.

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

lib/worktree.ts

@@ -39,6 +39,8 @@ export interface HarvestResult {
 function copyDirSync(src: string, dest: string): void {
   fs.mkdirSync(dest, { recursive: true });
   for (const entry of fs.readdirSync(src, { withFileTypes: true })) {
+    // Skip symlinks to avoid infinite recursion (e.g., .claude/skills/gstack → repo root)
+    if (entry.isSymbolicLink()) continue;
     const srcPath = path.join(src, entry.name);
     const destPath = path.join(dest, entry.name);
     if (entry.isDirectory()) {