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_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")
 }