Problem: unused variables (undo_state, sha_result, date match
captures) and duplicate if/else branches in gcal sync triggered
selene warnings and errors.
Solution: prefix unused state with underscore, simplify date
validation to a single pattern match, remove dead sha_result
call, and merge duplicate event deletion branches into a single
condition.
Problem: need to push tasks with due dates to Google Calendar
as all-day events.
Solution: add gcal module with OAuth 2.0 loopback flow, PKCE,
token refresh, calendar creation, and event CRUD. Maps pending
tasks to all-day events, deletes events on completion, and
stores event IDs in task _extra for round-trip.
