Use Agents SDK for durable agents guide (#28060)

* Use Agents SDK for durable agents guide

Revamp the durable-agents guide to use the Agents SDK and demonstrate real-time, bidirectional Workflows. Changes include: reformatted imports and npm install command to include `agents`; replace the previous Workflow example with a ResearchWorkflow implementing AgentWorkflow; add a ResearchAgent sample (src/agent.ts) to manage WebSocket/HTTP lifecycle and receive progress callbacks; update src/index.ts to route agent requests, expose /research and /status endpoints, and export agent/workflow classes; update wrangler config examples to register the ResearchAgent and ResearchWorkflow and migrations; modify .env, dev, and deploy instructions; add real-time client integration and a React useAgent example; and various wording/formatting improvements throughout the doc to explain Agents + Workflows integration.

* Update src/content/docs/workflows/get-started/durable-agents.mdx

Co-authored-by: mia303 <mmalden@cloudflare.com>

* Remove <Steps> tags and normalize code blocks

Remove custom <Steps> wrapper tags around example sections and replace step lists with plain headings (e.g., "Create src/workflow.ts", "Create src/agent.ts", "Replace src/index.ts"). Normalize fenced code block indentation and clean up surrounding whitespace for more consistent MDX rendering.

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Co-authored-by: mia303 <mmalden@cloudflare.com>

Author: threepointone