ci: isolate retarget run checks from master commits #1165

Retarget validation in #1161 dispatches workflow definitions from master so older PR branches can use the new reporting inputs and helper code. GitHub associates a workflow_dispatch run's native job checks with the dispatched ref's tip, even when the job checks out a pinned PR merge for testing. Consequently, a failing PR can add failing native job checks to a master commit. Dedicated e2e-status and Run Integration Tests checks on the PR head remain separate and accurate.

Review finding: #1161 (comment)
GitHub event semantics: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#workflow_dispatch

#1161 labels dispatched runs with the PR number to make their purpose explicit. Fully isolating the native checks requires a different execution/ref strategy; simply dispatching to the PR branch would use older workflow definitions that may not understand the new inputs.

Acceptance:

• Retarget tests and manual backfills work for PRs whose feature branch predates the CI fix.
• Native job results from PR validation do not misrepresent master's own validation.
• Required PR checks retain the correct head and tested merge identity, including failures and retries.
• Preserve metadata-edit behavior and read-only cache access.

This is a nonblocking follow-up to #1161; no app behavior changes are needed.