Continuation runs: re-enter an existing target branch

Controller/entry-point support for continuation runs: a new run pointed at an existing target branch pulls it (instead of creating it) and is told it continues prior work — internally identical to "stage 2 of a workflow."

Cut-line note: high demo value, not load-bearing — first to slip if the sprint math fails.

Done when:
- An E2E run against an existing branch resumes from its head and pushes on top of the prior work.

Depends on: none.

Start at the controller entry point for continuation runs and trace how a target branch is created or selected. Run or add the E2E scenario for an existing target branch; done means the run resumes from that branch's head and pushes new work on top of it.

Written by the indexing model from the issue text.