Add AGENTS.md and copilot-setup-steps.yml workflow

[Copilot]

Plan to add lead AGENTS.md and copilot-setup-steps.yml:

• Create AGENTS.md file with project overview, structure, and development guidelines
• Create .github/workflows/copilot-setup-steps.yml workflow file
• Verify files are properly created and formatted
• Fix Deno type check error in base64.ts fromString function
• Commit changes

Summary

Added two new files based on the hexagon/croner dev branch and fixed a pre-existing TypeScript type error:

1. AGENTS.md - Agent guide with project documentation
2. .github/workflows/copilot-setup-steps.yml - GitHub Copilot setup workflow
3. Fixed type error - Changed fromString to use .buffer property when calling fromArrayBuffer to fix TS2345 error

The type error was pre-existing in the main branch (also failing on CI there) where TextEncoder().encode() returns Uint8Array but fromArrayBuffer() expects ArrayBuffer. The fix accesses the underlying buffer via .buffer property.

Original prompt

Add a lead AGENTS.md and check dev branch of hexagon/croner on how to setup a copilot-setup-steps.yml

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Hexagon]

@copilot deno type check in ci failed

[Copilot]

@copilot deno type check in ci failed

Fixed in commit 2ff8e20. The issue was a pre-existing type error where TextEncoder().encode() returns Uint8Array but fromArrayBuffer() expects ArrayBuffer. Changed to use .buffer property to access the underlying ArrayBuffer.