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.

internal/cmdbin/exec_test.go

@@ -0,0 +1,34 @@
+package cmdbin
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRootHonorsKBROOT(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("KB_ROOT", dir)
+	if got := Root(); got != dir {
+		t.Fatalf("Root() = %q, want %q", got, dir)
+	}
+}
+
+func TestExecFileMissingIs127(t *testing.T) {
+	t.Setenv("KB_ROOT", t.TempDir())
+	if code := ExecFile("no/such-tool", nil); code != 127 {
+		t.Fatalf("exit = %d, want 127", code)
+	}
+}
+
+func TestExecFileRuns(t *testing.T) {
+	root := t.TempDir()
+	script := filepath.Join(root, "echo.sh")
+	if err := os.WriteFile(script, []byte("#!/bin/sh\nexit 3\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("KB_ROOT", root)
+	if code := ExecFile("echo.sh", nil); code != 3 {
+		t.Fatalf("exit = %d, want 3", code)
+	}
+}