_boot_in_flight gated the on-demand boot on installed_engine() with the
default "auto", not the configured local_runtime.backend. With no GPU
vendor detected, auto resolves to the CPU build only, so a user on
`backend: vulkan` with only the Vulkan engine installed never got a boot
from `hermes chat` ("The local model server isn't running"), while
ensure_local_runtime itself would have booted that exact engine. Found
during the live run for this PR.
ffbecdca20