fix(hooks): preserve mode on invalid arguments

[l2yujw]

Summary

Unsupported /ponytail mode arguments currently replace the active session mode with the configured default.

For example, when the active mode is ultra and the configured default is lite:

/ponytail ulta

silently changes the session to lite.

This PR makes unsupported arguments a no-op and adds a regression test proving that the active mode is preserved.

Root cause

The command parser assigned getDefaultMode() to every unsupported argument:

} else {
  mode = getDefaultMode();
}

The normal mode-write path then persisted that fallback as if the user had explicitly selected it.

Fix

Unsupported arguments now leave mode unset.

The existing downstream if (mode) guard prevents output and state mutation, while valid modes and bare status queries keep their existing behavior.

Verification

node --test tests/hooks.test.js
npm test
git diff --check