Add --repeat recurrence support for add/edit (closes #104)

[rameshbaskaran]

Closes #104.

What

Adds EKRecurrenceRule-backed recurrence support:

• reminders add <list> <reminder> --repeat daily|weekly|monthly|yearly
• --repeat-interval N to repeat every N units instead of every 1 (default 1), e.g. --repeat-interval 2 --repeat weekly for fortnightly.
• --repeat-until <date> to set a recurrence end date; omitting it repeats forever, matching Reminders.app's own default.
• reminders edit <list> <index> --repeat ... replaces an existing repeat rule; --clear-repeat removes it entirely.
• --format json output now includes recurrence, recurrenceInterval, and recurrenceEnd fields when present.

Why hourly is rejected rather than supported

EKRecurrenceFrequency has no hourly case -- Reminders.app's own UI doesn't expose hourly repeat either. Rather than silently degrade --repeat hourly to a daily rule (which would be a functional lie about what was actually configured), it's rejected at validate() with an explanation. Happy to change this if there's a preferred approach (e.g. a documented "hourly" alias that installs a daily rule with a note), but silent degradation felt worse than an explicit error.

Testing

Tested live against real Reminders.app data on macOS: add with daily/weekly/monthly + custom interval + end date, edit to both replace and clear a repeat rule, hourly correctly rejected with the explanatory error, --repeat-interval/--repeat-until correctly rejected when used without --repeat, and the --repeat + --clear-repeat conflict correctly rejected.

Added Tests/RemindersTests/RecurrenceTests.swift covering the frequency mapping, interval, end-date, and representability logic at the unit level.

Note on swift test locally: my dev machine currently only has Command Line Tools installed (no full Xcode), so swift test can't link XCTest here (xcrun --find xctest fails) -- this affects the pre-existing NaturalLanguageTests.swift identically, confirming it's a local environment gap and not something this PR introduces. swift build succeeds cleanly. The new test file is plain XCTest and should run normally in CI or on a machine with full Xcode -- I'm installing full Xcode now and will confirm swift test passes once that's done, but wanted to open this for review in the meantime rather than block on it.

Backward compatibility

All new flags are optional with sensible defaults (--repeat-interval defaults to 1, omitting --repeat/--repeat-until leaves existing behavior untouched). No changes to existing flag behavior or output format for reminders without a recurrence rule.

[rameshbaskaran]

Update: installed full Xcode and re-ran swift test on this branch.

• RecurrenceTests: 11/11 pass.
• NaturalLanguageTests: 4 failures, but confirmed these are pre-existing on unmodified main too (reproduced identically by checking out main and running the same suite) — a timezone-string-rendering difference on this machine (Asia/Dubai locale, weekOfYear field ordering in the Calendar description), unrelated to this PR's changes.

So the earlier note about not being able to run swift test locally is resolved, and the new recurrence tests pass cleanly. Also did a fresh live smoke test against real Reminders.app with the built binary (reminders add ... --repeat weekly --format json) — recurrence field correctly set and readable back.