chore: T8937: move conflict labeling+comment from eps1lon GHA to Mergify (#2103)

Removes the eps1lon `check-open-prs-conflict.yml` caller (which ran
`eps1lon/actions-label-merge-conflict` on push to base branches to label
conflicting PRs + comment) and replaces it with Mergify:

- Conflict LABEL (`conflicts`): already handled by the inherited central
  `Label conflicting pull requests` rule (predicate `conflict`, toggle).
  Mergify re-evaluates continuously, so the GHA's push-triggered re-scan
  is covered natively — no local rule needed for the label.
- Conflict COMMENT: a local additive rule `Comment on conflicting pull
  requests` (distinct name → does not replace the inherited label rule)
  posts the contributor-facing "please resolve" notice once when a PR
  enters conflict (Mergify dedupes the comment per rule+message).

The GHA's clean-side "conflicts resolved" comment is intentionally NOT
replicated — Mergify has no "was-conflicting-now-clean" predicate, and
keying on `-conflict` would match every non-conflicting PR.

This is the final piece of the T8937 Mergify-GHA retirement (the central
`check-pr-conflict.yml` reusable was preserved pending this consumer; it
is deleted in a follow-up vyos/.github PR once this merges).

Advances: T8937

🤖 Generated by [robots](https://vyos.io)

2 files changed, 26 insertions, 17 deletions

diff --git a/.github/mergify.yml b/.github/mergify.yml
index 89d61a85..97d6e462 100644
--- a/.github/mergify.yml
+++ b/.github/mergify.yml
@@ -21,7 +21,33 @@
 # The central config no longer enforces strict `T<digits>:` format — that
 # moved per-product-repo — so docs (a non-product repo) inherits only the
 # generic loose-title + body checks, no override needed.
+#
+# T8937 (2026-06): replaced the eps1lon `check-open-prs-conflict.yml` GHA
+# caller (which ran `eps1lon/actions-label-merge-conflict` to label + comment
+# on conflicting PRs) with Mergify. The inherited central rule `Label
+# conflicting pull requests` already toggles the `conflicts` label; the local
+# additive rule below restores the contributor-facing "please resolve" comment
+# the GHA used to post. (The GHA's clean-side "conflicts resolved" comment has
+# no Mergify equivalent — there is no "was-conflicting-now-clean" predicate —
+# so it is intentionally not replicated.)
 
 extends: mergify
 merge_protections_settings:
   reporting_method: check-runs
+
+pull_request_rules:
+  # Additive (distinct name from the inherited `Label conflicting pull
+  # requests` rule, so both fire): post a one-time comment when a PR enters
+  # conflict, mirroring the retired eps1lon GHA's dirty-side notice. Mergify's
+  # comment action dedupes per rule+message, so it posts once when the PR
+  # becomes conflicting, not on every re-evaluation.
+  - name: Comment on conflicting pull requests
+    description: Notify the author when their PR develops a merge conflict.
+    conditions:
+      - conflict
+      - '-closed'
+    actions:
+      comment:
+        message: >-
+          @{{author}} this pull request has conflicts. Please resolve them
+          before we can evaluate the pull request.
diff --git a/.github/workflows/check-open-prs-conflict.yml b/.github/workflows/check-open-prs-conflict.yml
deleted file mode 100644
index 9e850045..00000000
--- a/.github/workflows/check-open-prs-conflict.yml
+++ /dev/null
@@ -1,17 +0,0 @@
-name: "Open PRs Conflicts checker"
-on:
-  push:
-    branches:
-      - rolling
-      - sagitta
-      - circinus
-  workflow_dispatch:
-
-permissions:
-  contents: read
-  pull-requests: write
-
-jobs:
-  check-pr-conflict-call:
-    uses: vyos/.github/.github/workflows/check-pr-conflict.yml@production
-    secrets: inherit