docs(agents): document pi() as the Pi Durable agent

[eersnington]

The Agents docs describe the one pi() agent, which runs Pi Durable.

• One page. pi-durable.mdx is merged into pi.mdx. The sidebar has one "Pi Durable" entry that opens /agents/docs/pi, and every page calls the agent pi().
• No sandbox first. The Quickstart agent has no sandbox: its files live in the actor's database. A sandbox is an optional step, and the Sandboxes page starts with the no-sandbox case.
• Plain limits. Skills and AGENTS.md files are not supported yet, and the pages give a workaround: prompt sections and instructions. Human in the Loop builds its approval flow from an approval actor, because Pi has none built in.
• Pi Durable is in beta. The Pi page says so in a note. No other page has a beta label or migration content.
• pi-ai. The Pi page and Custom Tools show where pi-ai fits: Type for tool parameters, message types for conversation.context, and thinking levels.
• Examples. All of examples/docs uses the new API, with Pi packages at ^1.0.4. Examples that followed Cloudflare's or Pi Durable's own docs are replaced with original ones: a Pokédex tool for pi-ai, a preview deploy task, and a Portal 2 test chambers document. Other new examples: pi/messages.ts, pi/cost.ts, sandboxes/no-sandbox.ts, built-in-tools/reviewer.ts.
• Your own actions. The Pi page shows a one-call c.pi.usage cost action first, then a document example in three files. The document actions get the conversation from c.pi.root(), because the root conversation doesn't exist until something asks for it.
• Short answers stream. A short answer can arrive whole in message_end, with no text_delta before it. The streaming examples print it there, and the Pi, Client SDK, and Session Lifecycle pages say so.
• Links. External links open in a new tab.

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This is part 2 of 2 in a stack:

•  2  docs(agents): document pi() as the Pi Durable agent #336 👈
•  1  feat(pi): make pi() the Pi Durable agent #335