Add --repeat flag for recurring reminders

[rameshbaskaran]

Problem

There's currently no way to create or edit a recurring reminder through this CLI, even though:

• EKRecurrenceRule is public EventKit API (not a private/undocumented framework)
• Reminders.app itself fully supports repeat rules on reminders

Users migrating recurring tasks from other tools (Todoist, etc.) into Reminders via this CLI have had to flatten everything into one-shot due dates.

Changes

• New Recurrence: ExpressibleByArgument (Sources/RemindersLibrary/Recurrence.swift) parsing daily / weekly / monthly / yearly, plus interval forms like 2-weeks or 3-months.
• reminders add ... --repeat <freq> — requires --due-date to also be set (a recurrence rule has no meaning without a due date on EKReminder).
• reminders edit ... --repeat <freq> [--due-date <date>] — adds/replaces recurrence on an existing reminder; if the reminder has no due date yet, --due-date must be passed alongside --repeat.
• Recurrence rules are attached via addRecurrenceRule() before the initial EKEventStore.save() — adding them after save silently fails to persist (undocumented but consistently reported behavior across EventKit).
• Recurrence surfaced in both plain ((repeats: weekly)) and JSON ("recurrence": "weekly") output for show / show-all, so pre-existing recurring reminders (created via Reminders.app) now also report their recurrence.
• Unit tests for the Recurrence parser (Tests/RemindersTests/RecurrenceTests.swift).
• README section with usage examples.

Testing

Built and manually verified end-to-end against a real local Reminders list:

$ reminders add Test "Recurrence smoke test" --due-date "2026-08-10 09:00" --repeat weekly --format json
{
  ...
  "recurrence" : "weekly",
  ...
}
$ reminders show Test
6: Recurrence smoke test (in 1 week) (repeats: weekly)

Also verified edit --repeat adds recurrence to an existing non-recurring reminder, and that pre-existing recurring reminders created via Reminders.app UI (e.g. daily/weekly ones already on my list) correctly show (repeats: ...) with the new encoder logic — confirming the JSON/plain output changes don't regress anything for reminders this tool didn't create.

swift build -Xswiftc -warnings-as-errors passes clean locally. swift test requires a full Xcode toolchain (not available in my sandboxed environment, XCTest module missing from Command Line Tools only) — added tests should run fine under the macos-14 + Xcode 16.2 CI config already in .github/workflows/swift.yml.

Scope

This intentionally covers frequency + interval only (not day-of-week sets, end conditions/count, etc.) to keep the CLI surface simple — matches the level of expressiveness most task-migration use cases need. Happy to extend if there's interest.