feat(ci): queue mergify PRs with a queue ready label (#135)

Applying the label kicks a stuck PR without an @mergifyio comment. Auto-queue and review/CI gates stay.

.mergify.yml

@@ -1,6 +1,7 @@
 merge_queue:
   mode: serial
   max_parallel_checks: 1
+  queue_controls_comment: false
 
 merge_protections_settings:
   auto_merge_conditions:
@@ -29,3 +30,11 @@ queue_rules:
       title: inherit
       body: empty
     branch_protection_injection_mode: queue
+
+pull_request_rules:
+  - name: queue on queue ready label
+    conditions:
+      - label = "queue ready"
+    actions:
+      queue:
+        name: default

README.md

@@ -24,6 +24,10 @@ Exactly four headings in order: `## Summary`, `## Validation`, `## Tests`, `## N
 
 The two sanctioned deploy paths are merge to `main` triggering the pipeline and `workflow_dispatch` on that same pipeline. No manual workstation deploys to production.
 
+### Merge queue
+
+Consumer repos extend `.mergify.yml` via `extends: .github`. Mergify still auto-queues when it is awake. To kick a stuck PR, apply the `queue ready` label. That does not bypass `ci / ci` or `APPROVED`. Do not use `queued`; Mergify applies that while a PR is in the queue.
+
 ## What's in here
 
 ### Reusable Workflows