ci: add workflow_call trigger to ci.yml so deploy.yml gate resolves

deploy.yml uses `uses: ./.github/workflows/ci.yml` as a pre-deploy gate.
Without workflow_call in the on: block, GitHub rejects the reference and
the auto-deploy errors on every push to main. No plugin code changed;
no version bump (CI-only change per CONVENTIONS).

Agent-Session: cowork-2026-06-17-ci-workflowcall

.github/workflows/ci.yml

@@ -4,6 +4,7 @@ on:
   pull_request: {}
   push:
     branches: [main]
+  workflow_call: {}
 
 # Required: reusable phpcs job declares contents: write for phpcbf auto-commit,
 # but GitHub enforces that the caller must also declare it at workflow level.