ci: replace individual fan-in with unified CI summary job

[vdemeester]

Changes

Add a ci-summary fan-in job to ci.yml that depends on all CI jobs
(build, unit-tests, e2e-tests). This provides a single CI summary
check for branch protection, instead of listing every individual job.

The job runs with if: always() and treats success and skipped
as passing states, failing only on failure or cancelled.

Also adds merge_group trigger for merge queue support and updates the
concurrency group to include merge_group.head_ref.

Submitter Checklist

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

• Docs included if any changes are user facing
• Tests included if any functionality added or changed
• Follows the commit message standard
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  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 (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• 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]

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[vdemeester]

/kind cleanup