feat: artifact writer accepts any data type with auto-serialization

[krishna9358]

Summary

• artifact-writer.ts : modified the input schema from z.string() to z.any() and added logic to automatically JSON.stringify any content that isn't a string.

Why:
This allows users to connect list, object, or any other data types directly to the artifact writer without manual conversion, fixing the "Type mismatch" error shown in the screenshot.

Testing

• bun run test
• bun run lint
• bun run typecheck
• Additional notes:

Documentation

• Updated the relevant doc(s) (see docs/guide.md) or checked that no updates are needed.
• Recorded contract/architecture changes in both public docs and .ai logs when applicable.

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