feat(agenda): query the agenda window as JSON for external renderers

I need the agenda drawable from outside Emacs, on a 24-hour axis that
refreshes every few minutes.

- cj/agenda-window-json returns every entry intersecting an epoch-second
  window.
- Epoch seconds keep timezone out of the interface. The consumer converts
  per city, so DST weeks stop being a special case.
- Repeating entries expand to one row per occurrence through org's own
  arithmetic, so they land where my agenda shows them.
- Given an out-path it writes atomically. A reader on a timer never sees a
  half-written file.
- It skips archived and commented subtrees, as the agenda does.

The trap: SCHEDULED and DEADLINE are properties on the planning element,
not children in the parse tree. Mapping over 'timestamp returns neither,
and does it silently. An agenda is mostly made of those two. The tests pin
it with a fixture carrying no body timestamp.

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