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/kb/watch.go

@@ -1,5 +1,5 @@
 //usr/bin/env go run "$0" "$@"; exit
-// bin/kb/watch.go - re-index the 2dph brain when corpus files change.
+// bin/kb/watch.go — deprecated. Use bin/brain/watch.go.
 //
 // Usage:
 //