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.go

@@ -0,0 +1,54 @@
+package cmdbin
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+// Root is the 2dph checkout (KB_ROOT, or walk up for .git / var).
+func Root() string {
+	if v := os.Getenv("KB_ROOT"); v != "" {
+		return v
+	}
+	wd, err := os.Getwd()
+	if err != nil {
+		return "."
+	}
+	for i := 0; i < 10; i++ {
+		if _, err := os.Stat(filepath.Join(wd, ".git")); err == nil {
+			return wd
+		}
+		if _, err := os.Stat(filepath.Join(wd, "var")); err == nil {
+			return wd
+		}
+		parent := filepath.Dir(wd)
+		if parent == wd {
+			break
+		}
+		wd = parent
+	}
+	return "."
+}
+
+// ExecFile runs repo-relative path (python/bash shebang scripts) with stdio.
+func ExecFile(rel string, args []string) int {
+	path := filepath.Join(Root(), filepath.FromSlash(rel))
+	cmd := exec.Command(path, args...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Dir = Root()
+	if err := cmd.Run(); err != nil {
+		if ee, ok := err.(*exec.ExitError); ok {
+			return ee.ExitCode()
+		}
+		if errors.Is(err, os.ErrNotExist) || strings.Contains(err.Error(), "no such file") {
+			return 127
+		}
+		return 1
+	}
+	return 0
+}

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)
+	}
+}