feat: bin/brain/serve.go; search backend is Go not Python (#9)

* 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.

bin/brain/serve.go

@@ -0,0 +1,26 @@
+//usr/bin/env go run -tags=brain_serve "$0" "$@"; exit
+//go:build brain_serve
+//
+// bin/brain/serve.go - HTTP API for the 2dph brain.
+//
+//	KB_ROOT=/path/to/2dph ./bin/brain/serve.go
+//	KB_SEARCH_CMD=... KB_WORKERS=4 KB_PORT=8630 ./bin/brain/serve.go
+//
+// Default search backend is var/bin/brain-search (Go), not Python.
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/internal/httpapi"
+)
+
+func main() {
+	if os.Getenv("KB_ROOT") == "" {
+		if wd, err := os.Getwd(); err == nil {
+			os.Setenv("KB_ROOT", wd)
+		}
+	}
+	httpapi.Run()
+}