feat: brain/index.go shebang; mail import is not a brain write (D14). (#12)

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/doc.go

@@ -1,3 +1,3 @@
-// Commands in this directory are shebang mains (search.go).
-// search.go is behind the system_ladybug build tag (cgo).
+// Commands in this directory are shebang mains (search.go, serve.go, index.go,
+// get.go, stats.go, eval.go, watch.go), each behind an exclusive build tag.
 package main