docs: add custom digest function guide

[khaliqgant]

Summary

• add a user-facing custom digest functions guide covering the function contract, local test loop, CLI deployment, SDK deployment, runtime behavior, and M1 limits
• link the guide from README, getting started, cloud integration, and the TypeScript SDK README

Validation

• git diff --check origin/spec/workspace-primitives...HEAD
• verified linked local doc targets exist

Docs-only follow-up after #158.

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🧪 Generate unit tests (beta)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.