fix(ci): skip dist/ validation on push to main

The auto-bump workflow already rebuilds and commits dist/ on main pushes.
Running the validation in CI on the same event causes a race condition
where CI fails before auto-bump has a chance to commit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: leonardocouy

.github/workflows/main.yaml

@@ -41,6 +41,7 @@ jobs:
           python-version: '3.12'
 
       - name: Validate dist/ is up to date
+        if: github.event_name == 'pull_request'
         run: |
           python3 scripts/build_plugins.py
           if ! git diff --quiet dist/ .claude-plugin/marketplace.json; then