Port from Kilo-Org/kilocode#11240. Their issue #11227 lost a user's entire
working directory: a built-in-skill sentinel location resolved to the server
cwd and the skill-removal endpoint ran a recursive delete on it.
Hermes' /skills uninstall path (skills_hub.py) is already hardened, but the
agent-facing skill_manage(action='delete') path did a bare
shutil.rmtree(skill_dir) with no last-line validation. Add _validate_delete_target():
refuse to rmtree a path that (1) isn't strictly inside a known skills root,
(2) is a skills root itself, or (3) is reached via a symlink/junction.
Tests: 4 cases (normal delete works; symlinked dir, skills-root, out-of-tree
all refused). E2E verified with real symlink + file I/O.
