feat: brain/index.go shebang; mail import is not a brain write (D14).

Commands live at bin/brain/{index,get,stats,eval,watch}.go and
bin/mail/import.go, bin/markdown/import.go, bin/postgres/query.go.
Python remains the Ladybug write worker. index_mail is a deprecation
shim that rebuilds via --with-mail.

bin/brain/index.go

@@ -0,0 +1,24 @@
+//usr/bin/env go run -tags=brain_index "$0" "$@"; exit
+//go:build brain_index
+//
+// bin/brain/index.go - rebuild the Ladybug graph (Python write path).
+//
+//	./bin/brain/index.go --rebuild
+//	./bin/brain/index.go --rebuild --with-mail
+//	./bin/brain/index.go --dry-run --with-mail
+//
+// v1 write is always a rebuild when mail is included (live FTS/HNSW + bulk
+// insert corrupts Ladybug 0.19 WAL). `add` is v2.
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/internal/cmdbin"
+)
+
+func main() {
+	args := append([]string{"--with-mail"}, os.Args[1:]...)
+	os.Exit(cmdbin.ExecFile("bin/kb/index", args))
+}