refactor(backend): verb-first names for the provisioning modules

Rename the module files to match their functions' verb-first names:
`bottle_provision.py` -> `provision_bottle.py`,
`gateway_provision.py` -> `provision_gateway.py` (and the test file to match).
Imports, docstrings, and the git_gate/service.py pointer updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/firecracker/gateway_transport.py

@@ -4,7 +4,7 @@
 How the launcher stages files + runs commands in the running gateway: the
 docker exec/cp equivalents over SSH (dropbear + the stable infra key). The
 backend-neutral provisioning logic that drives it lives in
-`backend.gateway_provision`.
+`backend.provision_gateway`.
 """
 
 from __future__ import annotations