ci(validate): trigger on .github/policy/** so policy-prompt PRs clear the required check

A PR touching only .github/policy/** matched none of the validate
pull_request paths, so the required 'validate' check never ran via
pull_request and sat Expected forever (a workflow_dispatch check run
isn't associated with the PR, so it can't satisfy the gate). Mirrors
the existing .github/workflows/** carve-out.

.github/workflows/validate-plugins.yml

@@ -14,6 +14,11 @@ on:
       # check runs aren't associated with the PR, so they don't satisfy it). Run
       # validate on workflow changes too so those PRs can clear the gate in-context.
       - '.github/workflows/**'
+      # Same rationale for the scan policy prompt: a policy-only PR (.github/policy/**)
+      # touches none of the plugin paths above, so validate would never trigger via
+      # pull_request and the required check would sit "Expected" forever (a dispatch
+      # check run isn't associated with the PR, so it can't satisfy the gate either).
+      - '.github/policy/**'
   push:
     branches: [main]
     paths: