fix(cli): allow unauthenticated access to help

[dvd233]

What changed

Allow mmx help to run before authentication. The command only prints static MiniMax documentation links and does not require credentials or an API request.

Add an offline regression test that runs mmx help without credentials and checks that it exits successfully with the documentation heading.

Why

Before this change, mmx help was omitted from NO_AUTH_SETUP, so an unauthenticated non-interactive invocation failed with No credentials found before reaching the help command.

Validation

• npx tsc --noEmit
• npx eslint src/ test/ (0 errors; one pre-existing warning in test/sdk/speech.test.ts)
• npx tsx src/main.ts help without credentials: exit 0 and documentation links printed
• bun test not run because Bun is unavailable in the validation environment

Fixes #259

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