all-day events can gain a day when the model passes an exclusive end date #279

Steps to reproduce

1. ask the assistant to schedule a multi-day all-day event, e.g. "schedule X in my calendar tomorrow and the day after, all day"
2. the model calls schedule_event with start_date and end_date
3. the all-day branch converts to RFC 5545 with DTEND = end_date + 1, per the inclusive contract in the docstring

Not deterministic, depends on what the model sends: most of the time it sends inclusive dates, but sometimes it emits DTEND-style (exclusive) values, likely after seeing calendar data elsewhere (search results, exports), which is exclusive.

Expected behaviour

a two-day request produces a two-day event regardless of which convention the model uses

Actual behaviour

In manual e2e on my instance (PR #257 review rounds): one-day requests consistently sent end_date == start_date and stored correctly. One two-day run sent end_date = start_date + 2, which stored a three-day event. So the inclusive convention holds most of the time but not always.

Came up in the #257 review (the end_date + 1 delta discussion) and in #277's "other findings" list. Dropping the +1 is not the answer, that breaks the majority inclusive case. Options worth discussing:

• normalize prompt-side: make the tool description carry a concrete two-day example, e.g. "for an event on October 15 and 16, pass start_date 2026-10-15 and end_date 2026-10-16", models follow examples much better than contract wording
• or have the agent prompt state the convention explicitly when calendar tools are enabled