ci(mergify): fix auto-queue - migrate to merge_protections_settings/auto_merge_conditions (deprecation 2026-07-16)

The `pull_request_rules` `queue` action no longer auto-queues PRs in current
Mergify: a green, matching PR just reported "Merge queue is ready - use
`@Mergifyio queue`" and sat there, never merging. Automatic queueing now lives
in `auto_merge_conditions` under `merge_protections_settings`; the old
`autoqueue`/queue-action auto path is deprecated and stops working 2026-07-16.

Replace the non-functional `pull_request_rules` block with
`merge_protections_settings.auto_merge_conditions`, mirroring the exact same
gating conditions the old queue action used (base = main, -draft, -conflict,
label != broken, check-success = CI passed).

This changes ONLY the trigger (manual -> automatic). It does not touch the
queue's merge semantics: queue_rules (batch_size 1, merge_method merge,
merge_conditions), merge_queue (max_parallel_checks 1), and priority_rules
(P0-P9) are all unchanged. require-up-to-date stays ON - only batching
(batch_size > 1) would force that GitHub checkbox off, and we keep batch_size 1.
When a merge queue is configured, a matched PR is auto-queued (not merged
directly), so it still goes through the serial queue, is updated onto latest
main, re-runs CI, and merges on the real green "CI passed".

Structure verified against the live Mergify docs (file-format, merge-queue
rules/priority/lifecycle/batches, merge-protections auto-merge). YAML parses.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.mergify.yml

@@ -6,9 +6,17 @@
 #  Spec:   docs/ci/mergify-integration-spec.md + docs/ci/mergify.yml.proposed
 #          (issues #407 / #408).
 #  Schema: https://docs.mergify.com/configuration/file-format/
-#          Verified against the LIVE Mergify docs on 2026-07-06 (queue rules,
-#          the queue action, priority rules, parallel checks, batches, setup and
-#          lifecycle pages) — the config format evolves, so this is not from memory.
+#          Verified against the LIVE Mergify docs on 2026-07-07 (file-format, queue
+#          rules, priority, merge-queue lifecycle/setup/batches, and the
+#          merge-protections auto-merge pages) — the config format evolves, so this is
+#          not from memory.
+#          2026-07-07 CHANGE: auto-queueing migrated OFF the `pull_request_rules`
+#          queue-action path (which no longer auto-queues — a green matching PR just
+#          reported "Merge queue is ready — use `@Mergifyio queue`" and sat there) ONTO
+#          `merge_protections_settings.auto_merge_conditions` (see that block below).
+#          The old `autoqueue`/queue-action auto path is DEPRECATED and "will stop
+#          working on 2026-07-16" (docs.mergify.com/merge-queue/rules). This changes only
+#          the TRIGGER; the queue's merge semantics (below) are untouched.
 # ============================================================================
 #
 #  WHAT THIS DOES
@@ -125,22 +133,39 @@ priority_rules:
     priority: 1000
 
 # ---------------------------------------------------------------------------
-# pull_request_rules — WHICH PRs enter the queue.
-# The `queue` action is what actually ADDS a PR to the merge queue: per
-# docs.mergify.com/merge-queue/lifecycle, queue_conditions alone do NOT auto-queue a PR —
-# a queue action (or an `@mergifyio queue` command / auto_merge) is required, otherwise the
-# "Mergify Merge Queue" check sits permanently pending. A PR is queued as soon as it is green
-# on "CI passed", targets `main`, is not a draft, has no conflicts, and is not flagged
-# `broken`. `broken` / `draft` PRs are never queued.
+# merge_protections_settings — WHICH PRs are AUTOMATICALLY added to the queue.
+#
+# This REPLACES the old `pull_request_rules` `queue` action. That action no longer
+# auto-queues in current Mergify: a green, matching PR just reported "Merge queue is
+# ready — use `@Mergifyio queue`" and sat there forever (never merged). Automatic
+# queueing now lives in `auto_merge_conditions` under `merge_protections_settings`. The
+# old `queue_rules[].autoqueue` field (and the queue-action auto path) is DEPRECATED and
+# "will stop working on 2026-07-16. Use `auto_merge_conditions` in
+# `merge_protections_settings` instead" (docs.mergify.com/merge-queue/rules).
+#
+# `auto_merge_conditions` accepts `true` (auto-queue every mergeable PR) or, as here,
+# "a list of conditions to restrict the audience" (docs.mergify.com/configuration/
+# file-format). We give the SAME set the old queue action used, so EXACTLY the same PRs
+# auto-queue: green on "CI passed", targeting `main`, not a draft, no conflicts, not
+# `broken`.
+#
+# WHY THIS PRESERVES require-up-to-date: this changes only the TRIGGER (manual →
+# automatic). It does NOT touch how the queue validates or merges — batch_size 1,
+# merge_method merge, and max_parallel_checks 1 above are unchanged — and those are the
+# settings that interact with require-up-to-date (only BATCHING, batch_size > 1, forces
+# that checkbox OFF; see the invariants header + docs.mergify.com/merge-queue/batches).
+# When a merge queue is configured, a matched PR is auto-QUEUED, not merged directly:
+# "Every PR is auto-queued. The merge queue then handles routing and merging"
+# (docs.mergify.com/merge-protections/auto-merge) — so it still goes through the serial
+# queue, gets updated onto the latest `main`, re-runs CI, and merges on the real green
+# "CI passed". Mergify also auto-reads GitHub branch protection (the required "CI passed"
+# check + require-up-to-date) and injects it as a merge condition, so the GitHub gate is
+# enforced on top of queue_rules.merge_conditions.
 # ---------------------------------------------------------------------------
-pull_request_rules:
-  - name: Queue green, non-draft, non-conflicting PRs targeting main
-    conditions:
-      - base = main
-      - -draft
-      - -conflict
-      - label != broken
-      - check-success = CI passed
-    actions:
-      queue:
-        name: default
+merge_protections_settings:
+  auto_merge_conditions:
+    - base = main
+    - -draft
+    - -conflict
+    - label != broken
+    - check-success = CI passed