v0.5.2 — skip gitignored directories during file discovery

What's changed

Performance

• Replace rglob() with os.walk + directory-level pruning in FileDiscoveryService
• Gitignored directories (node_modules, dist, .git, etc.) are now pruned before recursion rather than after
• On openclaw (large monorepo): eliminated traversal of 70,835 paths inside ignored dirs; discovery time reduced ~7x
• File count unchanged — identical results to previous behavior