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/markdown/doc.go

@@ -0,0 +1,3 @@
+// Commands in this directory are shebang mains (import.go), tagged so
+// `go build ./bin/markdown` does not see two mains.
+package main