feat(hyperguild): subcommand router skeleton

Lays down the cmd/hyperguild/ entry point. Defines the subcommand
contract (ctx, args, stdin, stdout, stderr) error, the dispatch()
function that's testable without os.Exit, and stubs for tier / brain /
mode that return errNotImplemented. Subsequent commits replace each
stub.

Part of Plan 4 (hyperguild CLI) of the hyperguild migration.

cmd/hyperguild/main_test.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDispatch_Help_PrintsUsageAndReturnsZero(t *testing.T) {
+	var out, errBuf bytes.Buffer
+	code := dispatch(context.Background(), []string{"--help"}, strings.NewReader(""), &out, &errBuf)
+	assert.Equal(t, 0, code)
+	assert.Contains(t, out.String(), "Usage: hyperguild")
+	assert.Contains(t, out.String(), "tier")
+	assert.Contains(t, out.String(), "brain")
+	assert.Contains(t, out.String(), "mode")
+}
+
+func TestDispatch_NoArgs_PrintsUsageAndReturnsTwo(t *testing.T) {
+	var out, errBuf bytes.Buffer
+	code := dispatch(context.Background(), []string{}, strings.NewReader(""), &out, &errBuf)
+	assert.Equal(t, 2, code)
+	assert.Contains(t, errBuf.String(), "Usage: hyperguild")
+}
+
+func TestDispatch_UnknownSubcommand_ReturnsTwo(t *testing.T) {
+	var out, errBuf bytes.Buffer
+	code := dispatch(context.Background(), []string{"bogus"}, strings.NewReader(""), &out, &errBuf)
+	assert.Equal(t, 2, code)
+	assert.Contains(t, errBuf.String(), "unknown subcommand: bogus")
+}
+
+func TestDispatch_KnownSubcommand_RoutesAndReturnsExitCode(t *testing.T) {
+	var out, errBuf bytes.Buffer
+	code := dispatch(context.Background(), []string{"tier"}, strings.NewReader(""), &out, &errBuf)
+	// At Task 1 time, tier returns the not-implemented error → exit 1.
+	assert.Equal(t, 1, code)
+	assert.Contains(t, errBuf.String(), "not implemented")
+}