fix(calendar): document the real create_event target-calendar behaviour

[JulianFriedle]

Summary

do_manage_calendar resolves the target calendar for create_event from calendar_href or calendar, by id, id prefix or case-insensitive name. The JSON schema described calendar_href as a "Specific calendar URL" and
calendar as a list-only filter — neither matches the implementation. The "defaults to first calendar" part was accurate but incomplete, and that is where the harm is: _ensure_default_calendar() returns the owner's first calendar row, which on any install where a local calendar was created before a CalDAV one is the local one. The event is then written with
caldav_sync_pending = NULL, writeback_event() skips it as "not a caldav calendar", and the tool returns exit_code=0. The user is told the event was created and it never appears in their calendar. A value that matches no calendar falls back to the same default without an error, so a wrong calendar name is indistinguishable from success. This PR corrects both field
descriptions and states the consequence of omitting the target. Behaviour is unchanged; the tests pin the existing resolution rules so the schema and the implementation cannot drift apart again.

Target branch

• This PR targets dev, not main. All PRs land in dev; main is curated by the maintainer at each release.

Linked Issue

Fixes #6286

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end.
• I did not run the app/runtime validation and stated that gap in How to Test.

How to Test

1. Reproduce on dev

On an install where a local calendar exists and was created before the CalDAV one — the default if you opened the calendar page before connecting CalDAV — ask the agent in agent mode to create an event without naming a calendar, then:

select c.name, c.source, e.caldav_sync_pending, e.remote_href
from calendar_events e
join calendars c on c.id = e.calendar_id
where e.summary = '<your event>';

Returns the local calendar with caldav_sync_pending = NULL and remote_href = NULL. The tool reported exit_code=0, but nothing appears in the remote calendar.

2. With this change

The schema now states that the field takes an id, an id prefix or a name, that omitting it falls back to the owner's first calendar, and what that costs. Passing the intended calendar explicitly:

{"action": "create_event", "summary": "Launch", "dtstart": "2026-10-01T10:00:00",
 "calendar_href": "<calendar name>"}

puts the event on the CalDAV calendar, caldav_sync_pending becomes create, and the push runs.

3. Unit tests

python3 -m pytest tests/test_calendar_target_calendar.py -q

Expected: 6 passed — resolution by name, by id prefix, via the calendar alias, case-insensitive name matching, the first-calendar fallback, and the silent fallback on an unmatched name.

Note that these tests pass on dev as well: the resolution logic was always correct, only its documentation was not. They are a regression guard against the schema and the implementation drifting apart again, not a proof of the fix.

4. Runtime validation performed

Docker install, Ollama + qwen3:8b and qwen3:14b, CalDAV against Google Calendar. Before: agent-created events landed in the local Personal calendar with caldav_sync_pending = NULL and never reached Google, three attempts in a row, each reported as successful. After: with the target calendar passed explicitly the event is created on the CalDAV calendar and queued for push.

Visual / UI changes — REQUIRED if you touched anything that renders

Not applicable — this PR touches src/tool_schemas.py and adds tests/test_calendar_target_calendar.py. No rendering code, no CSS, no HTML, no SVG, no static/js/.

• I am not an LLM agent submitting a bulk PR.