docs: Update Workflows integration documentation for Agents

Comprehensive update to the Workflows integration documentation to reflect
the new AgentWorkflow base class and enhanced integration features added in
cloudflare/agents PR #799.

Key additions:
- AgentWorkflow base class with typed Agent access
- Automatic workflow tracking in SQLite
- Lifecycle callbacks (onWorkflowProgress, onWorkflowComplete, etc.)
- Approval workflow methods (approveWorkflow, rejectWorkflow)
- State synchronization between workflows and agents
- Query and cleanup methods for workflow management
- Common patterns (background processing, human-in-the-loop, retries)

Synced from: cloudflare/agents#799

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: github-actions[bot]