run_meta_lock relocked each cascade agent's `agent-<name>` input and only
then fast-forwarded `applied/<name>/main` to the forge config main. The
declared input has been the applied mirror (`git+file://<applied>`) since
b9cef950, so the relock pinned the mirror's old head; the cascade's
rebuilds run with `relock = false` and build that lock. An agent whose
mirror was behind forge main was redeployed on its old config, and the
mirror was then moved to the new head, claiming the agent current.
Fast-forward first, then relock. The fast-forward stays best-effort per
agent: a failed one logs a warn and that agent is relocked against
whatever its mirror has. The two steps move into catch_up_cascade_agents,
which takes them as injected effects (the advance_applied_main pattern in
actions.rs) so the order is unit-tested without nix or a forge.
The stale comment claiming the relock ran against "the forge URL" is
replaced by the actual constraint on the helper's doc.
Closes#5090
971426a1c0