feat: skip all CI checks and comment once when PR has merge conflicts

[rnetser]

Problem

When a pushed PR has merge conflicts, the webhook server still queues and runs all CI checks (tox, pre-commit, container build, python module install, conventional-title, security checks, custom checks). Running CI against an unmergeable state wastes compute, and the author gets no clear signal that they need to resolve conflicts.

Behavior

• On push/synchronize, determine conflict state via pull_request.mergeable is False (existing detection in label_pull_request_by_merge_state).
• If the PR has conflicts:
  • Add the has-conflicts label (existing behavior).
  • Skip all CI checks — do not queue or run any of them.
  • Post a comment explaining why CI did not run and that resolving the conflicts (rebase/merge) will re-trigger checks on the next push.
• Dedup via the has-conflicts label (no spam):
  • The first conflicting push adds the label (absent→present) → post the comment.
  • Subsequent conflicting pushes see the label already present → skip CI, no new comment.
• When conflicts clear: the has-conflicts label is removed (existing behavior) and CI runs normally on the next push. If conflicts later reappear, the label was removed in between, so a fresh comment posts.

Done

• process_opened_or_synchronize_pull_request skips all CI check queueing and runner tasks when pull_request.mergeable is False
• has-conflicts label added on first conflicting push (existing transition logic reused)
• Comment posted exactly once per conflict episode, gated on the has-conflicts label absent→present transition (present = already notified, no re-comment)
• Comment text explains CI was skipped due to merge conflicts and that resolving them re-triggers checks
• Unit tests: (a) conflicting push with no label → skips CI + adds label + comments; (b) conflicting push with label already present → skips CI, no new comment; (c) no-conflict push → CI runs normally
• docs/ regenerated via docsfy (or deferred to a follow-up issue if docsfy is unavailable)

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Assisted-by: PI (gpt-5.6-luna)