fix(skills): return None instead of truthy stub when skill load fails

build_skill_invocation_message() returns a non-empty placeholder string
('[Failed to load skill: ...]') when the skill exists in the command cache
but loading the actual SKILL.md payload fails. CLI/gateway callers treat
any truthy return value as success, so the failure is silently routed into
the model as if it were a valid skill prompt.

Return None instead, matching the existing behavior for unknown commands,
so callers using 'if msg:' can properly detect the failure.

agent/skill_commands.py

@@ -425,7 +425,7 @@ def build_skill_invocation_message(
 
     loaded = _load_skill_payload(skill_info["skill_dir"], task_id=task_id)
     if not loaded:
-        return f"[Failed to load skill: {skill_info['name']}]"
+        return None
 
     loaded_skill, skill_dir, skill_name = loaded