Migrate Sheets auth from an API key to a service-account credential
(GOOGLE_CREDENTIALS_FILE) with the read+write spreadsheets scope, add a
WriteSheet primitive, and use it to actively manage empty duty weeks in
the weekly reminder.
Each fire now handles a fixed this-week/next-week model (replacing the
DUTY_AHEAD_DAYS announce):
- this week empty -> pick the two people with the fewest duties (random
tie-break), write their names into cols B/C, and announce
- this week filled -> announce who is on duty
- next week empty -> ask people to volunteer before it is auto-assigned
Col G ("Liczba dyżurów") is a live formula, so names-only writes keep the
fewest-duties selection fair across weeks. Selection, row tracking, range
building, and every fire() branch are unit-tested; live tests for sheet
read/write are env-gated.
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