Arbeitszeit: Arbeitstag beginnen, beenden, aendern und loeschen

beginneArbeitstag legt per saveSchicht.php eine laufende Schicht fuer
beliebige Mitarbeiter an (der Menuepunkt "Arbeitstag beginnen" wirkt nur
fuer den angemeldeten Benutzer). Damit kann auch eine Pause die erste
Buchung des Tages sein. Zeitbuchungen lassen laufende Schichten offen
und ziehen nur den Beginn bei Bedarf vor.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -105,7 +105,9 @@ The store can read both the connector format and the older Telefonbuch cookie-ar
 - `getMitarbeiter(suchstring?, mitarbeitergruppe?)` reads `/module/mitarbeiter/livesearch.php` and returns employee ID, number, last name, first name, and active flag.
 - `getTaetigkeiten()` reads `/module/arbeitszeit/taetigkeiten-livesearch.php` (active and inactive) and returns activity ID, UUID, number, name, active flag, area (`bereichId`, `bereich`) and the flags `maschineZuordnen`, `abwesenheit`, `notizErzwingen`.
 - Agrarmonitor has no cost center master data (`Kostenstelle` is only a free-text field on machines, employees and articles), so there is no `getKostenstellen()`. See `docs/zeitbuchung-analyse.md`.
-- `createZeitbuchung(daten)` books an activity block (`taetigkeitId`, optional `maschinenId`, `notiz`, `kostenstelle`) or a break (`art: 'pause'`) for `mitarbeiterId` from `start` to `ende` (minute precision, Europe/Berlin) and returns `{ ref }`. The first booking of a day creates the shift via `nachtragen.php`; later bookings go into the existing shift, which is extended if needed. Breaks need an existing shift. Agrarmonitor has no cost center field, so `kostenstelle` is appended to the note as `Kostenstelle: …`. Pass `datum` (YYYY-MM-DD) for blocks after midnight that belong to the previous workday.
+- `beginneArbeitstag(mitarbeiterId, start, { datum? })` starts the workday: creates a running shift (attendance without end) for any employee and returns `{ ref }` (`schicht:<mitarbeiterId>:<YYYY-MM-DD>:<id>`). If a running shift already exists that day, its ref is returned instead. The Agrarmonitor menu link "Arbeitstag beginnen" (`bueroAnundAbmelden.php`) only works for the logged-in user, so the connector uses `module/auftraege/api/saveSchicht.php`.
+- `beendeArbeitstag(ref, ende)` ends the shift, `updateArbeitstag(ref, { start, ende })` corrects it (`ende: null` keeps it running), `deleteArbeitstag(ref)` deletes the shift including all activities and breaks.
+- `createZeitbuchung(daten)` books an activity block (`taetigkeitId`, optional `maschinenId`, `notiz`, `kostenstelle`) or a break (`art: 'pause'`) for `mitarbeiterId` from `start` to `ende` (minute precision, Europe/Berlin) and returns `{ ref }`. Bookings go into the shift of that day (started with `beginneArbeitstag`), which is extended if needed; a running shift stays open. Without any shift, an activity creates one via `nachtragen.php`, a break is rejected. Agrarmonitor has no cost center field, so `kostenstelle` is appended to the note as `Kostenstelle: …`. Pass `datum` (YYYY-MM-DD) for blocks after midnight that belong to the previous workday.
 - `updateZeitbuchung(ref, daten)` changes times, activity, machine and note of a booking (the old note is replaced); changing the type (activity/break), employee or workday is rejected.
 - `deleteZeitbuchung(ref)` deletes a booking; deleting the last activity of a shift also deletes the shift in Agrarmonitor.
 - Refs look like `verteilung:<mitarbeiterId>:<YYYY-MM-DD>:<id>` or `pause:…`. Rejections by Agrarmonitor (HTTP 4xx such as 422 with the reason in the body) and invalid input throw `ZeitbuchungError` (`grund`, `status`); network and 5xx errors are thrown unchanged so callers can retry.