Follow-up on the salvaged #55614. The PR added host-based routing to
_create_backend (precedence: oss > host > platform) but two sibling surfaces
didn't mirror it:
- system_prompt_block() checked host before oss, so an oss+host config ran
OSS but told the model it was self-hosted HTTP. Reordered to match routing.
- Platform setup (hermes memory setup mem0 --mode platform) left a stale host
in mem0.json; since host beats platform, the user kept routing to the
self-hosted server. save_config merges (no delete), so clear host to ""
rather than pop() so the merge actually overwrites it.
Adds regression tests for both (mutation-checked).
53edf6f983