ci: stop the merge gatekeeper waiting on Graphite's mergeability check

[asaf-sw]

Stops the merge gatekeeper timing out on every stacked PR. The cause is Graphite / mergeability_check, which never completes while a PR's base is another open PR, so the gate waits out its full hour and dies reporting zero failed jobs.

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Detailed Summary for AI Bots

Problem

Failed job count:      0
Error: context deadline exceeded

It self-resolves as the stack merges bottom-up, which is worse than a hard failure: it trains reviewers to ignore a red gate.

Root cause

Diffing the gate's own "All jobs" and "Completed jobs" groups across all 120 polls of run 31624863250 leaves exactly one job that never completes:

last All jobs: 16   last Completed: 15
NEVER COMPLETED -> ['Graphite / mergeability_check']

The correlation with base branch is exact: #14938 and #14946 (base main) pass; #14944, #14947 and #14948 (stacked) time out. Skipped checks were a red herring, since #14944 and #14947 had zero skips.

Change

Add Graphite / mergeability_check to the gate's ignored list on the pull_request step. It is a stack-state indicator, not a CI signal about the code, and the list already ignores code-review/reviewable for the same reason. The merge_group step is untouched, since Graphite does not post this check against a merge queue ref.

Name matching is substring-based, as the single build_docker_images entry covering all three matrix variants shows.

[reviewable-StarkWare]

This change is