Add Flow Cron / scheduled tx patterns to Cadence skill

[peterargue]

Summary

Add guidance and patterns for Flow Cron (scheduled transactions) to the Cadence skill.

Context

Scheduled transactions are a Flow-specific capability that LLMs don't understand well out of the box. The Cadence skill should include rules and examples for writing and deploying scheduled transactions using Flow Cron.

Acceptance Criteria

• Flow Cron patterns and best practices added to the Cadence skill
• Includes example scheduled transaction templates
• Covers common use cases (periodic state updates, maintenance tasks, etc.)

References

• Discussed in AI tooling sync (Mar 25)
• Flow x AI doc: "Make sure it covers scheduled Txs (Flow Cron)"