* feat(brain): HTTP serve from bin/brain/serve.go, default Go search binary.
Move the HTTP package to internal/httpapi. Default backend is
var/bin/brain-search, not Python. bin/serve.go stays as a deprecation shim.
* feat(httpapi): default search backend is var/bin/brain-search.
bin/brain/serve.go is the command; bin/serve.go stays as a tagged
deprecation shim. Tests fail if the default path still names Python.
