Add a German prompt that prepares a school day

Tagesvorbereitung is where the two systems meet. It attaches the day's
timetable and then asks for the rest: match each lesson to its Schulcloud
course, read what is new there, look at what the previous lesson covered,
check both lists of due work, and read the notes on the periods, where the
announced tests are. Written for the morning before school, so it asks for
something short.

The argument takes heute, morgen, übermorgen or a date in either notation,
because evening preparation is the normal case and a German keyboard writes
21.09.2026. Registered only with WebUntis configured: a Tagesvorbereitung
that has to ask which lessons exist is not one.

readTimetable is shared with untis_timetable, the way readCourse is shared
with get_course, so an attached day reads exactly like a fetched one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/smoke.mjs

@@ -549,6 +549,14 @@ if (hasUntis) {
 		huge.text.split('\n')[0],
 	);
 
+	const prep = await client.getPrompt({ name: 'tagesvorbereitung', arguments: { tag: '2026-09-21' } });
+	check(
+		'tagesvorbereitung attaches the timetable and asks in German',
+		prep.messages.length === 2 &&
+			/## Montag, 21\.09\.2026/.test(prep.messages[0].content.text) &&
+			/Bereite mich auf den Schultag/.test(prep.messages[1].content.text),
+		prep.description,
+	);
 }
 
 console.log('\n== api_get guard rails ==');