Enable merge queue support in GitHub workflows (#2433)

## Summary
- Configures Mergify merge queue with automatic DCO sign-off to resolve
DCO check failures on merge commits
- Removes GitHub native merge queue triggers from all workflows
- Adds auto-merge rule for PRs with `ready` label and required approvals

## Problem
The DCO (Developer Certificate of Origin) GitHub App was failing on
merge commits created by GitHub's native merge queue, as those commits
lacked the required `Signed-off-by:` trailer.

## Solution
Switch to Mergify's merge queue which automatically adds DCO sign-off to
all merge commits it creates.

## Changes
### Mergify Configuration (`.github/mergify.yml`)
- Added `queue_rules` with automatic DCO sign-off in commit messages
- Added auto-merge rule that queues PRs when:
  - Label `ready` is applied
  - 2+ approvals received
  - All required checks pass (DCO, tests, quality, etc.)

### Workflow Updates (removed `merge_group` triggers)
- `.github/workflows/ready-label-check.yaml`: Removed merge_group
trigger
- `.github/workflows/test-check-transformers.yaml`: Removed merge_group
trigger and condition
- `.github/workflows/test-check.yaml`: Removed merge_group trigger
- `.github/workflows/quality-check.yaml`: Removed merge_group trigger
- `.github/workflows/linkcheck.yml`: Removed merge_group trigger

## Testing
After merging, GitHub's native merge queue should be disabled in
repository settings and Mergify will handle all merge queue operations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: dsikka

.github/mergify.yml

@@ -1,4 +1,43 @@
+queue_rules:
+  - name: default
+    merge_method: merge
+    commit_message_template: |
+      {{ title }} (#{{ number }})
+
+      {{ body }}
+
+      Signed-off-by: Mergify <noreply@mergify.com>
+    queue_conditions:
+      - check-success=DCO
+      - check-success=quality-check
+      - check-success=transformers-tests
+      - check-success=base-tests (3.10)
+      - check-success=base-tests (3.13)
+      - check-success=pytorch-tests (3.10)
+      - check-success=pytorch-tests (3.13)
+      - check-success=markdown-link-check
+
 pull_request_rules:
+  - name: Automatically merge when ready
+    conditions:
+      - base=main
+      - label=ready
+      - "#approved-reviews-by>=2"
+      - check-success=DCO
+      - check-success=quality-check
+      - check-success=transformers-tests
+      - check-success=base-tests (3.10)
+      - check-success=base-tests (3.13)
+      - check-success=pytorch-tests (3.10)
+      - check-success=pytorch-tests (3.13)
+      - check-success=markdown-link-check
+      - check-success=ready-label-check
+      - -conflict
+      - -draft
+    actions:
+      queue:
+        name: default
+
   - name: label-documentation
     description: Automatically apply documentation label
     conditions: