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