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/docker-entrypoint

@@ -2,10 +2,10 @@
 # bin/docker-entrypoint - run 2dph tools inside the container.
 #
 #   brain                 shell (default)
-#   brain search <q>      bin/kb/search
-#   brain index           bin/kb/index
-#   brain watch <dir>     watchdog re-indexer (bin/kb/watch)
-#   brain serve           async Go HTTP server (bin/serve)
+#   brain search <q>      bin/brain/search.go
+#   brain index           bin/kb/index --with-mail
+#   brain watch <dir>     compiled /app/bin/watch (bin/brain/watch.go)
+#   brain serve           compiled /app/bin/serve (bin/brain/serve.go)
 #   brain extract         bin/facts/extract (docker×compose pairing)
 #   brain audit           bin/facts/audit
 #
@@ -18,7 +18,7 @@ shift || true
 case "$CMD" in
   shell) exec bash ;;
   search) exec "$KB_PY" /app/bin/kb/search "$@" ;;
-  index) exec "$KB_PY" /app/bin/kb/index "$@" ;;
+  index) exec "$KB_PY" /app/bin/kb/index --with-mail "$@" ;;
   watch) exec /app/bin/watch "$@" ;;
   serve) exec /app/bin/serve "$@" ;;
   extract) exec "$KB_PY" /app/bin/facts/extract "$@" ;;