Move the HTTP package to internal/httpapi. Default backend is var/bin/brain-search, not Python. bin/serve.go stays as a deprecation shim.
Collapse nested kbsearch/chats go.mod into the root module. Ranking stays cgo-free under internal/brain/rank so CI does not need ladybug. bin/kb/search is a deprecation wrapper that still sets CGO and builds the binary.
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