1. Vision SSRF (#2878, #2872):
Add _validate_url_safe() that resolves the target hostname via DNS and
rejects any IP in private (RFC1918), loopback, link-local, or reserved
ranges before requests.get() is called. This blocks attacks that use
attacker-controlled image URLs to probe internal services or cloud
metadata endpoints (169.254.169.254).
2. Skill install path traversal (#2873):
Add _safe_skill_dir() that validates the skill name cannot escape the
skills/ root directory. Rejects names containing '..', absolute paths,
and any resolved path that falls outside the custom_dir boundary.
Applied to _add_url(), _add_package(), and delete().
Both fixes include comprehensive unit tests (19 test cases) covering
blocked patterns, edge cases, and allowed legitimate usage.
Closes#2878Closes#2873
Ref: #2872
