ci: add CI gate workflow

[anacrolix]

Closes #19909.

Summary

Switches CI from individual per-workflow required checks to a single CI Gate / ci-gate aggregator. This is required for clean handling of merge queues and PRs simultaneously — without a gate, there's no single check to require on merge_group events, and managing required checks means touching GitHub settings rather than code.

The gate currently requires exactly the same jobs that were previously required, so this is a one-to-one transition:

• Lint / lint
• All tests / tests-mac-linux (ubuntu-24.04, macos-15, windows-2025)
• All tests (with -race) / ...
• Test Hive / test-hive (...)
• Consensus spec / ...
• Check large files / check
• Benchmarks / benchmarks
• Kurtosis Assertoor GitHub Action / assertoor_{regular,pectra}_test
• Manifest Check / ManifestCheck
• Reproducible build / reproducible-build (...)
• QA - RPC Integration Tests (Gnosis) / gnosis-rpc-integ-tests
• QA - RPC Integration Tests Remote / mainnet-rpc-integ-tests-remote

Each called workflow gains workflow_call: and loses its pull_request: trigger — the gate owns PR and merge-queue coverage. Standalone push, schedule, and workflow_dispatch triggers are preserved.

Running different jobs on PR vs merge queue

Because the gate triggers on both pull_request and merge_group, it's now straightforward to run a lighter suite on PRs and the full suite only when merging:

# ci-gate.yml
expensive-job:
  if: github.event_name == 'merge_group'
  uses: ./.github/workflows/expensive.yml
  secrets: inherit

The aggregator ignores skipped jobs, so jobs conditional on event type don't block the gate in the other context. This is a follow-up opportunity — for now all jobs run on both.

How to merge

1. In branch protection for main, remove all individual required checks and add CI Gate / ci-gate
2. Merge this PR — the gate is now in effect for all subsequent PRs and merge queue entries

Other changes

• Deletes stub workflows ci.yml, test-win-downloader.yml, check.yml
• Fixes event_name-dependent conditions in test-hive.yml and test-kurtosis-assertoor.yml
• Upgrades check-large-files.yml checkout v4→v6
• Adds workflow_dispatch to test-bench.yml and test-integration-caplin.yml
• Removes redundant daily schedule from test-all-erigon.yml
• Documents workflow_call vs inline guidance in .github/README.md

[anacrolix]

Just trying to iron out why a few jobs won't run, I think it's a naming clash.

[anacrolix]

Ok, should be good. Add CI Gate to required checks. Remove the rest.

If upon adding to main, something breaks, or something unwanted runs, modify the ci-gate workflow to fix.

A small tweak to ci-gate workflow is then only step required to tune for merge queues, to come after.

[anacrolix]

@copilot fix the artifact upload for qa RPC by making it distinct from the other qa RPC workflow. Pls no mistakes

[Copilot]

@anacrolix I've opened a new pull request, #19959, to work on those changes. Once the pull request is ready, I'll request review from you.

[anacrolix]

All green