Add documentation for task management and workflow integration

Documents the new @task() decorator and workflow integration features
added in cloudflare/agents PR #683. Includes:

- Comprehensive guide for integrating Cloudflare Workflows with Agents
- Comparison of ctx.waitUntil() vs Workflow approaches
- Complete example with React client and real-time progress updates
- Configuration and deployment instructions

Related PR: cloudflare/agents#683

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: github-actions[bot]