…igontech#21445)

## Problem

The merge-queue fail-fast optimization (erigontech#20789) cancels the entire CI
Gate run as soon as one leaf job fails, so the gate doesn't stall ~30
min waiting for the heavy jobs (hive/kurtosis/eest) to finish. The
downside: `gh run cancel` cancels the *whole* run including the leaf
that called it, so the real culprit's conclusion flips `failure →
cancelled` — visually identical to every innocent sibling. The `ci-gate`
aggregator then lumps `failure` and `cancelled` together and prints only
job names, so finding the actual failure means drilling into step-level
conclusions by hand.

This surfaced when erigontech#21426 (a green, approved one-line PR) was silently
evicted from the merge queue: a flaky data race in
`TestHistoryVerification_SimpleBlocks` tripped the fast-cancel, but the
run showed a sea of "cancelled" jobs with no indication which one
failed.

## Fix

Keep the latency win; make the root cause prominent.

- **Each leaf** emits a GitHub `::error` annotation right before `gh run
cancel`. Only the *true trigger* reaches this step — collateral jobs
have it `skipped` by the in-progress cancellation — so the annotation is
attributed to the actual failing job and shows at the top of the run +
in the PR Checks tab.
- **The `ci-gate` aggregator** now names the root cause instead of
dumping ambiguous job names. It identifies the trigger precisely — the
job whose "Cancel workflow run on failure" step actually ran (`success`)
— and annotates it plus its failing step. Falls back to listing all
genuinely-failed jobs on `pull_request` runs (where no fast-cancel
fires).

## Validation

- `actionlint` clean on all changed files (no new findings; the
pre-existing shellcheck infos are untouched).
- Replayed the aggregator's query against the real failed run that
evicted erigontech#21426: it now outputs exactly
`::error … race-tests … (execution-other, serial) — failed step: Run
execution-other tests`,
isolating the real culprit and excluding the 4 collateral `hive-eest`
failures.

No behavior change to the fast-cancel itself — only added visibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: info@weblogix.biz <admin@10gbps.weblogix.it>