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,
 	}
 }