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/cron-cli/register.ts

@@ -1,6 +1,7 @@
 import type { Command } from "commander";
 import { formatDocsLink } from "../../terminal/links.js";
 import { theme } from "../../terminal/theme.js";
+import { applyParentDefaultHelpAction } from "../program/parent-default-help.js";
 import {
   registerCronAddCommand,
   registerCronListCommand,
@@ -24,4 +25,6 @@ export function registerCronCli(program: Command) {
   registerCronAddCommand(cron);
   registerCronSimpleCommands(cron);
   registerCronEditCommand(cron);
+
+  applyParentDefaultHelpAction(cron);
 }