feat(validation): sync CommonalitiesTest regression branches from Commonalities main

[hdamker]

What type of PR is this?

enhancement/feature

What this PR does / why we need it:

Adds a tooling-owned pipeline that keeps a dedicated CommonalitiesTest regression repository's regression/* branches in sync with Commonalities main and sweeps them for validation-rule regressions. Each run diff-checks Commonalities main against what's already synced; when code/common/ changed, it syncs it and cherry-picks the same commit onto every regression branch, and when the templates changed it regenerates one branch's API templates, then runs the existing regression-runner sweep against all of them. validation-regression.yml now also calls this pipeline, forced, on its existing regression-canary trigger.

• Automates a sync/cherry-pick/sweep loop that so far only existed as a one-time, hand-run bootstrap.
• Extends the tooling-push regression signal (catch a rule regression before release) to CommonalitiesTest, not just ReleaseTest.
• No new automation code needed in CommonalitiesTest itself.

Which issue(s) this PR fixes:

Fixes #

Special notes for reviewers:

New workflow files can't be exercised via workflow_dispatch until merged to main; validated locally with pytest (new unit tests on the diff/copy logic) and actionlint. The job that writes to CommonalitiesTest (diff, sync, cherry-pick, template regen) mints a camara-release-automation App token rather than camara-validation's, since the validation App currently has no contents permission.

Changelog input

 release-note
Add a tooling-owned pipeline to keep CommonalitiesTest's regression branches synced with Commonalities and swept for validation-rule regressions.

Additional documentation

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docs