Add e2e-checkpoint-store workflow (manual, dispatch-only)

A workflow_dispatch workflow to run the e2e suite against a chosen checkpoint backend (git-branch or git-refs) for a chosen agent. Split into its own PR so it lands on the default branch and becomes dispatchable: workflow_dispatch only shows the "Run workflow" button once the file is on the default branch.

Dispatch-only — merging triggers nothing automatically, so it cannot affect CI or other workflows. Selecting git-refs is meaningful once the git-refs backend lands (#1566); until then dispatch it with ref set to that branch to exercise the backend pre-merge.