docs(validation): align regression-testing.md with the single-branch model

[hdamker]

What type of PR is this?

documentation

What this PR does / why we need it:

validation-framework was retired into main on 2026-05-05 (#260), but validation/docs/regression-testing.md still described ReleaseTest's canary as targeting that branch. Corrected every claim to match the current single-branch model (release_automation/docs/branching-model.md): ReleaseTest's caller pins tooling_ref_override: main — a literal name, not a resolved SHA, which is also why context.json's tooling_ref comes through unset for ReleaseTest captures — now documented explicitly.

Also removed two stray internal references left in docstrings, and a dead validation-framework push trigger left over in validation-settings-ci.yml.

Which issue(s) this PR fixes:

N/A — standalone documentation correction, no linked issue.

Special notes for reviewers:

Verified against a live capture run against camaraproject/ReleaseTest before writing the correction (tooling_ref omitted from the fixture, as now documented). actionlint and the release_automation test suite still pass.

Changelog input

release-note
Aligns regression-testing.md with the current single-branch model; no functional change.

Additional documentation

This section can be blank.

docs