feat(hyperguild): mode subcommand

'hyperguild mode <cloud|client-local|sovereign>' writes a per-mode
.mcp.json template:

  - cloud:        brain MCP only
  - client-local: brain + routing placeholder with _routing_pending
                  pointer to Plan 6
  - sovereign:    brain only + top-level _mode_note explaining Crush
                  is primary; .mcp.json is Claude Code fallback

Default output is ./.mcp.json; --out overrides; --force overwrites.
Brain URL sourced from BRAIN_URL (default http://koala:30330) so the
template stays in lockstep with the user's brain host.

All three subcommands now wired; notYet/errNotImplemented removed
from main.go.

cmd/hyperguild/main.go

@@ -4,7 +4,6 @@ package main
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -16,18 +15,11 @@ import (
 // touching os.Stdin / os.Stdout / os.Exit.
 type subcommand func(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error
 
-// errNotImplemented is returned by stub subcommands until their task lands.
-var errNotImplemented = errors.New("not implemented")
-
-func notYet(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
-	return errNotImplemented
-}
-
 func subcommands() map[string]subcommand {
 	return map[string]subcommand{
 		"tier":  runTier,
 		"brain": runBrain,
-		"mode":  notYet,
+		"mode":  runMode,
 	}
 }
 

cmd/hyperguild/main_test.go

@@ -33,9 +33,13 @@ func TestDispatch_UnknownSubcommand_ReturnsTwo(t *testing.T) {
 	assert.Contains(t, errBuf.String(), "unknown subcommand: bogus")
 }
 
-func TestDispatch_KnownSubcommand_RoutesAndReturnsExitCode(t *testing.T) {
+func TestDispatch_KnownSubcommand_RoutesToHandler(t *testing.T) {
+	// "mode" without args fails → exit 1, message on stderr.
+	// (Confirms dispatch reached the handler rather than printing "unknown
+	// subcommand: mode".)
 	var out, errBuf bytes.Buffer
 	code := dispatch(context.Background(), []string{"mode"}, strings.NewReader(""), &out, &errBuf)
 	assert.Equal(t, 1, code)
-	assert.Contains(t, errBuf.String(), "not implemented")
+	assert.Contains(t, errBuf.String(), "name required")
+	assert.NotContains(t, errBuf.String(), "unknown subcommand")
 }

cmd/hyperguild/mode.go

@@ -0,0 +1,99 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+)
+
+func runMode(ctx context.Context, args []string, _ io.Reader, stdout, stderr io.Writer) error {
+	fs := flag.NewFlagSet("mode", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	out := fs.String("out", ".mcp.json", "output file path")
+	force := fs.Bool("force", false, "overwrite an existing file")
+	// Pull the first positional (mode name) out so flags after it still parse
+	// with stdlib flag (which stops at the first non-flag arg).
+	if len(args) < 1 {
+		return errors.New("mode: name required (cloud|client-local|sovereign)")
+	}
+	name := args[0]
+	if err := fs.Parse(args[1:]); err != nil {
+		return fmt.Errorf("parse flags: %w", err)
+	}
+
+	brainURL := os.Getenv("BRAIN_URL")
+	if brainURL == "" {
+		brainURL = defaultBrainURL
+	}
+
+	var doc map[string]any
+	switch name {
+	case "cloud":
+		doc = modeCloud(brainURL)
+	case "client-local":
+		doc = modeClientLocal(brainURL)
+	case "sovereign":
+		doc = modeSovereign(brainURL)
+	default:
+		return fmt.Errorf("mode: unknown mode: %s (expected cloud|client-local|sovereign)", name)
+	}
+
+	if !*force {
+		if _, err := os.Stat(*out); err == nil {
+			return fmt.Errorf("mode: %s exists (use --force to overwrite)", *out)
+		}
+	}
+
+	body, err := json.MarshalIndent(doc, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal mode doc: %w", err)
+	}
+	if err := os.WriteFile(*out, append(body, '\n'), 0o644); err != nil {
+		return fmt.Errorf("write %s: %w", *out, err)
+	}
+	fmt.Fprintf(stdout, "wrote %s (mode: %s)\n", *out, name) //nolint:errcheck
+	return nil
+}
+
+func modeCloud(brainURL string) map[string]any {
+	return map[string]any{
+		"mcpServers": map[string]any{
+			"brain": map[string]any{
+				"url":         brainURL + "/mcp",
+				"description": "Brain MCP — knowledge query, write, ingestion, session log",
+			},
+		},
+	}
+}
+
+func modeClientLocal(brainURL string) map[string]any {
+	return map[string]any{
+		"mcpServers": map[string]any{
+			"brain": map[string]any{
+				"url":         brainURL + "/mcp",
+				"description": "Brain MCP — knowledge query, write, ingestion, session log",
+			},
+			"routing": map[string]any{
+				"url":              "http://koala:30310/mcp",
+				"description":      "Mode 2 routing pod — routes skill calls to LiteLLM/local",
+				"_routing_pending": "Plan 6 — routing pod not deployed yet; this URL is a placeholder",
+			},
+		},
+	}
+}
+
+func modeSovereign(brainURL string) map[string]any {
+	return map[string]any{
+		"_mode_note": "Sovereign mode primarily uses Crush + LiteLLM. This .mcp.json is provided as Claude Code fallback (e.g. emergency offline editing).",
+		"mcpServers": map[string]any{
+			"brain": map[string]any{
+				"url":         brainURL + "/mcp",
+				"description": "Brain MCP — knowledge query, write, ingestion, session log",
+			},
+		},
+	}
+}

cmd/hyperguild/mode_test.go

@@ -0,0 +1,123 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func readJSON(t *testing.T, path string) map[string]any {
+	t.Helper()
+	b, err := os.ReadFile(path)
+	require.NoError(t, err)
+	var out map[string]any
+	require.NoError(t, json.Unmarshal(b, &out))
+	return out
+}
+
+func TestRunMode_Cloud_Default(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+	t.Setenv("BRAIN_URL", "http://koala:30330")
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"cloud", "--out", outPath}, strings.NewReader(""), &stdout, &stderr)
+	require.NoError(t, err)
+
+	got := readJSON(t, outPath)
+	servers, ok := got["mcpServers"].(map[string]any)
+	require.True(t, ok, "mcpServers must be a JSON object")
+	assert.Contains(t, servers, "brain")
+	assert.NotContains(t, servers, "routing")
+	assert.NotContains(t, got, "_mode_note")
+}
+
+func TestRunMode_ClientLocal_HasRoutingPlaceholder(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+	t.Setenv("BRAIN_URL", "http://koala:30330")
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"client-local", "--out", outPath}, strings.NewReader(""), &stdout, &stderr)
+	require.NoError(t, err)
+
+	got := readJSON(t, outPath)
+	servers := got["mcpServers"].(map[string]any)
+	require.Contains(t, servers, "brain")
+	require.Contains(t, servers, "routing")
+
+	routing := servers["routing"].(map[string]any)
+	assert.Contains(t, routing, "_routing_pending")
+}
+
+func TestRunMode_Sovereign_HasModeNote(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"sovereign", "--out", outPath}, strings.NewReader(""), &stdout, &stderr)
+	require.NoError(t, err)
+
+	got := readJSON(t, outPath)
+	assert.Contains(t, got, "_mode_note")
+	servers := got["mcpServers"].(map[string]any)
+	assert.Contains(t, servers, "brain")
+	assert.NotContains(t, servers, "routing")
+}
+
+func TestRunMode_DefaultsOutToCwd(t *testing.T) {
+	dir := t.TempDir()
+	t.Chdir(dir) // Go 1.24+ — replaces the older os.Chdir-with-cleanup pattern
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"cloud"}, strings.NewReader(""), &stdout, &stderr)
+	require.NoError(t, err)
+	_, statErr := os.Stat(filepath.Join(dir, ".mcp.json"))
+	assert.NoError(t, statErr, ".mcp.json should exist in cwd")
+}
+
+func TestRunMode_UnknownMode(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"bogus", "--out", outPath}, strings.NewReader(""), &stdout, &stderr)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "unknown mode")
+}
+
+func TestRunMode_NoArgs(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{}, strings.NewReader(""), &stdout, &stderr)
+	assert.Error(t, err)
+}
+
+func TestRunMode_RefusesToOverwrite(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+	require.NoError(t, os.WriteFile(outPath, []byte(`{"existing":"file"}`), 0o644))
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"cloud", "--out", outPath}, strings.NewReader(""), &stdout, &stderr)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "exists")
+}
+
+func TestRunMode_Force(t *testing.T) {
+	dir := t.TempDir()
+	outPath := filepath.Join(dir, ".mcp.json")
+	require.NoError(t, os.WriteFile(outPath, []byte(`{"existing":"file"}`), 0o644))
+
+	var stdout, stderr bytes.Buffer
+	err := runMode(context.Background(), []string{"cloud", "--out", outPath, "--force"}, strings.NewReader(""), &stdout, &stderr)
+	require.NoError(t, err)
+	got := readJSON(t, outPath)
+	assert.Contains(t, got, "mcpServers")
+	assert.NotContains(t, got, "existing")
+}