fix(skills): replace deprecated utcnow in skill timestamp helper (#5777)

* fix(skills): replace deprecated utcnow in skill timestamp helper

_now_iso() builds the 'created' value in skill frontmatter. datetime.utcnow()
returns a naive datetime and has been deprecated since Python 3.12, scheduled
for removal. Switch to the timezone-aware datetime.now(timezone.utc), keeping
the serialized YYYY-MM-DDTHH:MM:SSZ shape unchanged so existing skill files
keep parsing.

timezone.utc is used rather than the datetime.UTC alias, which is 3.11+ only.

Adds regression tests covering the deprecation, the serialized shape, and
UTC correctness under a non-UTC local timezone -- the last guards against a
bare datetime.now(), which yields the same shape but local wall time.

Fixes #5697

* test(skills): skip timezone mutation where unsupported

---------

Co-authored-by: Alexandre Teixeira <alexandremagteixeira@gmail.com>

services/memory/skill_format.py

@@ -50,7 +50,7 @@ import json
 import logging
 import re
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any, Dict, List, Optional
 
 logger = logging.getLogger(__name__)
@@ -441,4 +441,4 @@ class Skill:
 
 
 def _now_iso() -> str:
-    return datetime.utcnow().strftime("%Y-%m-%dT%H:%M:%SZ")
+    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")