Fix CI workflow to run on all PRs

Remove the `branches: ["*"]` filter from the pull_request trigger.
The glob pattern `*` doesn't match `/`, so PRs targeting branches
with slashes (like feature/...) were not triggering the workflow.

By removing the filter, the workflow now runs on all PRs regardless
of target branch, while still not running on post-merge to main
(since there's no push trigger).

Author: claude

.github/workflows/validate.yml

@@ -2,7 +2,6 @@ name: Validate
 
 on:
   pull_request:
-    branches: ["*"]
 
 jobs:
   tests: