ci: run validate on workflow-file changes (fix required-check deadlock)

A PR touching only .github/workflows/** (e.g. an action-SHA re-pin like this one)
never matched validate-plugins.yml's pull_request path filter, so the required
'validate' check never ran and the PR sat 'Expected — Waiting for status to be
reported' with no way to clear it (workflow_dispatch check runs aren't associated
with the PR, so they don't satisfy the required check; the ruleset has no bypass
actors). Add .github/workflows/** to the trigger so workflow-only PRs validate
in-context and can clear the gate — and so this PR unblocks itself.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/validate-plugins.yml

@@ -8,6 +8,12 @@ on:
       - '*/agents/**'
       - '*/skills/**'
       - '*/commands/**'
+      # `validate` is a required status check, so a PR that touches ONLY workflow
+      # files (e.g. an action-SHA re-pin) would otherwise never trigger validate
+      # and sit "Expected — Waiting for status to be reported" forever (workflow_dispatch
+      # 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/**'
   push:
     branches: [main]
     paths: