WIP: Proposal: Configure merge_queue on the repository (to replace tide)

[vdemeester]

Changes

Summary

This PR proposes to add support for GitHub's merge queue to automate PR merging and ensure the main branch remains stable. The merge queue validates that approved PRs work correctly when merged with the latest main before allowing them to land.

• Updated workflows to trigger on merge_group events in addition to pull_request:
• Reduce the e2e matrix to the minimum when being run in the merge queue.
  This optimization reduces merge queue CI time and flakyness while maintaining safety, since PRs already pass the full matrix during review. It also helps not overriding to much the runners.
• Added comprehensive merge queue documentation to DEVELOPMENT.md

Testing

The workflow changes are backward compatible:

• Regular PR builds continue to work exactly as before
• The merge_group trigger is additive and only activates when merge queue is enabled
• The minimal parameter defaults to false, preserving full matrix behavior for existing callers

Next Steps

Before merging this PR, deciding whether we want to take this approach or not. This is a proposal/exploration, but we can definitely decide to keep using tide.

After merging this PR, repository administrators will need to:

1. Enable merge queue in branch protection settings for main
2. Configure merge queue parameters:
   • Merge method: rebase (matches current setup)
   • Build concurrency: 5
   • Batch size: 1-5 PRs
   • Timeout: 60 minutes
3. Coordinate with tektoncd org about Prow/Tide integration (if applicable)

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
To complete the pull request process, please ask for approval from vdemeester after the PR has been reviewed.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[vdemeester]

cc @afrittoli @aThorp96 @twoGiants @waveywaves