ci: support merge queues

[chaptersix]

Summary

• run Continuous Integration and Govulncheck for GitHub merge-group commits
• ensure Validate Server Version executes for merge groups instead of being skipped by its PR-specific condition
• verify the root and cliext Go modules remain tidy
• remove two stale ui-server/v2 v2.50.1 checksums exposed by the initial tidy run

Why

GitHub merge queues test synthetic merge_group commits. Required workflows that only listen for pull_request do not create checks for those commits, leaving queued pull requests blocked until timeout.

The server-version job also used github.base_ref, which is populated for pull requests but not merge groups. Explicitly accepting the merge_group event ensures the validation actually runs on the queued commit.

The tidy check provides additional protection for dependency updates that merge without a textual conflict but leave module metadata inconsistent.

Validation

• actionlint .github/workflows/ci.yaml .github/workflows/govulncheck.yml
• go mod tidy with a clean diff in the root module
• go mod tidy with a clean diff in cliext
• go test ./...
• end-to-end queue test in chaptersix-org/temporal-cli: GitHub created a gh-readonly-queue/main/... ref, triggered both workflows, ran Validate Server Version, and merged the test PR through the queue

Sandbox evidence:

• ci: test merge queue support chaptersix-org/temporal-cli#1
• https://github.com/chaptersix-org/temporal-cli/actions/runs/29164112024
• https://github.com/chaptersix-org/temporal-cli/actions/runs/29164112020

Administrator follow-up

After this lands on main, repository administrators can enable the merge-queue rule and select the intended required checks. Organization-managed checks such as CODEOWNERS, Opengrep, and CLA should be confirmed to support merge-group commits before they are made queue requirements.