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/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

bin/markdown/import.go

@@ -0,0 +1,20 @@
+//usr/bin/env go run -tags=markdown_import "$0" "$@"; exit
+//go:build markdown_import
+//
+// bin/markdown/import.go - split markdown into leafs (mistune).
+//
+//	./bin/markdown/import.go [dir]
+//	./bin/markdown/import.go --files a.md,b.md --json
+//
+// NOTE: never run `gofmt -w` on this file — it breaks the shebang.
+package main
+
+import (
+	"os"
+
+	"github.com/eSlider/2dph/internal/cmdbin"
+)
+
+func main() {
+	os.Exit(cmdbin.ExecFile("bin/md/import", os.Args[1:]))
+}