ci: only check index.json freshness on pull requests

The index-freshness gate raced update-index.yml. Both trigger on a push to
main touching skills/**, so a merge that adds a skill runs the check against
the pre-merge index.json while update-index.yml is regenerating it. Merging
#129 turned main red for about fourteen minutes before the next push cleared
it, with nothing actually wrong.

The check still does its job where it matters - on pull requests, where the
contributor is the one who has to regenerate. On main, update-index.yml is
the mechanism that keeps it current, so verifying it in parallel only ever
produces a false red.

.github/workflows/validate-skills.yml

@@ -47,7 +47,11 @@ jobs:
         run: python3 tools/validate-agentskills.py --strict
 
       # index.json is generated; a PR that changes a description must regenerate it.
+      # Pull requests only: on a push to main, update-index.yml regenerates
+      # index.json in parallel with this job, so checking here would race and go
+      # red on every merge that adds a skill before self-healing seconds later.
       - name: Check index.json is current
+        if: github.event_name == 'pull_request'
         run: python3 tools/generate-index.py --check
 
       # Description quality gate. Pre-existing failures are grandfathered in