Per the operator's ruling on #4851 (option (a)): the hive holds no
write identity on the forge, so the `push_config` mirror of an applied
repo's tags and `main` to `agent-configs/<name>` is removed, along with
its `run_config_push` helper and its call in `sync_agent`.
Docs and comments that described the mirror now say that forge `main`
advances only by an operator's merge.
Refs #4851
456a4511ab