The update skill no longer specifies pinned_at_utc's format or revisions.json's layout, which the audit builder reads unchecked

[realmarcin]

From the round-1 review of #265.

Where: .claude/skills/update-xmech-page/SKILL.md steps 1 and 7, and scripts/fleet/build_site_audit.py.

The step 7 rewrite dropped "pinned_at_utc (ISO, with its offset)". Step 1 says only to write the pins to $SNAP/revisions.json "with the pin time", and gives no layout. The builder now reads pins["mechs"][<Mech>]["sha" / "commit_date"], pins["claw"] and pins["pinned_at_utc"], and copies the pin time into the audit unchecked. A pin time with no offset, or unreadable, would only surface later as the nightly's AUDIT failure.

Fix:

• give the layout and the pin-time format in step 1;
• have the builder validate the pin time with check_cards.pin_time() and refuse one that is unreadable or in the future.