Add monthly scheduling with set week [1st|2nd|3rd|4th|last]

[dblock]

Adds set week [1st|2nd|3rd|4th|last] to S'Up on the nth weekday of every month. Use unset week to revert to the weekly schedule. When monthly mode is active, sup_every_n_weeks is ignored.

Usage

@sup set week 2nd    # S'Up on the 2nd Monday of every month
@sup set week last   # S'Up on the last Monday of every month
@sup set week        # show current setting
@sup unset week      # revert to weekly schedule

Changes

• lib/models/channel.rb: sup_week_of_month field (nil=weekly, 1–5), WEEK_OF_MONTH_ORDINALS lookup table, monthly_sup_date helper (handles "last" via end-of-month math), monthly-aware sup? and next_sup_at, display helpers sup_week_of_month_s and sup_schedule_s
• lib/commands/set.rb: set_week_of_month, unset_week_of_month, wired into channel_set/channel_unset, channel_info now uses sup_schedule_s for both weekly and monthly modes
• lib/api/presenters/channel_presenter.rb: exposes sup_week_of_month and sup_week_of_month_s
• lib/commands/help.rb: documented set week and unset week
• spec/models/channel_spec.rb: monthly sup? and next_sup_at tests including "last" weekday
• spec/slack-sup/commands/set_spec_channel.rb: view/change/cannot-change tests for week
• CHANGELOG.md: new entry