fix(cli): exit 0 when invoking parent commands without a subcommand (#73077)

Several `openclaw <parent>` commands (channels, plugins, approvals, devices,
cron, mcp) were exiting with code 1 when invoked bare, while printing the
same help-style content that `<parent> --help` produces (which exits 0).
This broke `&&` chains and surfaced a misleading
`ELIFECYCLE Command failed with exit code 1.` line under pnpm.

Add a small `applyParentDefaultHelpAction(cmd)` helper in
`src/cli/program/parent-default-help.ts` that attaches a default action
which prints the parent's own help and sets `process.exitCode = 0`. The
helper is a no-op when the parent already has its own action (e.g.
`agents` defaulting to `agents list`), so existing intentional defaults
are preserved.

Apply it to the six core parents listed in #73077.

src/cli/mcp-cli.ts

@@ -12,6 +12,7 @@ import {
   normalizeStringifiedOptionalString,
 } from "../shared/string-coerce.js";
 import { resolveGatewayAuthOptions } from "./gateway-secret-options.js";
+import { applyParentDefaultHelpAction } from "./program/parent-default-help.js";
 
 function fail(message: string): never {
   defaultRuntime.error(message);
@@ -147,4 +148,6 @@ export function registerMcpCli(program: Command) {
       }
       defaultRuntime.log(`Removed MCP server "${name}" from ${result.path}.`);
     });
+
+  applyParentDefaultHelpAction(mcp);
 }