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/postgres/doc.go

@@ -0,0 +1,2 @@
+// Commands in this directory are shebang mains (query.go).
+package main