[Docs] Document custom automation schedule presets

[roomote-community]

Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

Document the exact built-in schedule values accepted by manage_custom_automations, alongside its existing cron and natural-language schedule options.

Why this change was made

The public automation guide mentioned schedule management but did not name the presets exposed by the task tool, leaving its documented usage incomplete.

Impact

Users managing custom automations from Roomote tasks can now choose the correct preset values without guessing. There is no product runtime change.

[roomote-community]

No code issues found. See task

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