feat(admin): add onScheduleEffect wrapper for scheduled functions

[fwal]

Closes #60

Summary

Adds an onScheduleEffect wrapper around firebase-functions/v2/scheduler's onSchedule, so scheduled functions (cron jobs) can run inside Effect like the other triggers.

export const cleanup = onScheduleEffect(
  { runtime, schedule: 'every 24 hours' },
  (event) => Effect.log(`Running cleanup job: ${event.jobName}`),
);

Details

• Follows the existing wrapper conventions: runtime in the options object (matching onMessagePublishedEffect / onTaskDispatchedEffect, rather than the third-argument shape sketched in the issue), Effect.withSpan('onScheduleEffect') tracing, and defect logging via logger.error.
• Scheduled events carry no payload, so there is no schema overload — a single signature receiving the ScheduledEvent (jobName, scheduleTime).
• Exported from functions.ts, available as import { onScheduleEffect } from '@effect-firebase/admin'.
• Docs: new "Scheduled (onSchedule)" section in the admin README, plus example and API reference entries in the create-firebase-function skill.

Validation

• nx run @effect-firebase/admin:build ✅
• nx run @effect-firebase/admin:lint ✅
• nx run @effect-firebase/admin:test — 31/31 passing ✅
• Prettier check on changed files ✅

🤖 Generated with Claude Code

https://claude.ai/code/session_01AEWBZud3svoJnGZZUStRcu

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