feat: migrate from luxon to dayjs

[Copilot]

Replaces the luxon date/time library with dayjs across rita-core.

Changes

• Atom.ts: replaced DateTime.fromISO(val).toJSDate() with dayjs(val).toDate()
• Calculation.ts: replaced DateTime.fromJSDate(item).toISO() with dayjs(item).toISOString()
• DateCalculation.ts: replaced all DateTime/Duration usage with dayjs and the dayjs/plugin/duration plugin:
  • Duration.fromMillis(ms) → dayjs.duration(ms)
  • Duration.fromObject({[unit]: value}) → dayjs.duration({[unit]: value})
  • d.toMillis() → d.asMilliseconds()
  • DateTime.fromJSDate(date).plus(dur).toJSDate() → dayjs(date).add(dur).toDate()
  • DateTime.fromJSDate(date).minus(dur).toJSDate() → dayjs(date).subtract(dur).toDate()
  • res.as(unit) → unchanged (dayjs Duration has the same .as() API)
• Assertions.ts: replaced Duration.isDuration(value) with dayjs.isDuration(value)
• Test files (macros.ts, schema.test.ts, calculation.test.ts): replaced DateTime usage with dayjs
• package.json: removed luxon and @types/luxon, added dayjs ^1.11.20

Testing

All 98 existing tests pass with no modifications needed.