Add Agent class architecture documentation

[threepointone]

Summary

This PR adds comprehensive documentation explaining the layered architecture of the Agent class from the Agents SDK.

The new documentation covers:

• Layer 0: Durable Object - The base primitives (constructor, RPC, fetch, WebSockets, alarms, storage)
• Layer 1: PartyKit Server - The opinionated wrapper providing developer-friendly callbacks
• Layer 2: Agent - The full-featured agent implementation with state persistence, scheduling, MCP, email handling, and more

This helps developers understand how Agents are built on top of Durable Objects and PartyKit's Server class, empowering them to make the most of the SDK and avoid common pitfalls.

Source

Synced from cloudflare/agents PR #636

Test plan

• Documentation follows cloudflare-docs conventions
• All internal links are valid
• Code examples use TypeScriptExample component
• Content is placed in the appropriate location (/agents/concepts/)
• Frontmatter includes appropriate metadata

🤖 Generated with Claude Code

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/agents/	@irvinebroque, @rita3ko, @elithrar, @thomasgauvin, @threepointone, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://5a46b1d7.preview.developers.cloudflare.com
Preview Branch URL: https://sync-docs-pr-636.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/agents/concepts/agent-class/	https://sync-docs-pr-636.preview.developers.cloudflare.com/agents/concepts/agent-class/