refactor: one Go module; brain search in bin/brain + internal/brain. (#8)

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.

bin/brain/doc.go

@@ -0,0 +1,3 @@
+// Commands in this directory are shebang mains (search.go).
+// search.go is behind the system_ladybug build tag (cgo).
+package main

bin/brain/search.go

@@ -0,0 +1,23 @@
+//usr/bin/env go run -tags=system_ladybug "$0" "$@"; exit
+//go:build cgo && system_ladybug
+//
+// bin/brain/search.go - deduction search over the 2dph brain.
+//
+//	./bin/brain/search.go "query" [--root facts|info] [--repo P] [-n N] [--json]
+//	./bin/brain/search.go serve [port]
+//	./bin/brain/search.go --list-model
+//
+// Needs CGO + libladybug (CGO_CFLAGS/CGO_LDFLAGS). Prefer the wrapper
+// bin/kb/search which sets those and builds a binary for the embed daemon.
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/internal/brain"
+)
+
+func main() {
+	os.Exit(brain.Main(os.Args[1:]))
+}