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/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",
+			},
+		},
+	}
+}