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

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