Add Sessions, Sub-agents, and Think docs (#29882)

Add new API reference pages for Sessions, Sub-agents, and Think (persistent session storage, child Durable Object agents with RPC, and the Think opinionated chat framework). Update the Agents API index to include Sub-agents, Sessions, and Think. Clarify subagent delegation in chat-agents (note about in-process ToolLoopAgent vs Durable Object sub-agents). Rename/standardize chat recovery flag in durable-execution (unstable_chatRecovery → chatRecovery) and update related links/reference text. Also include minor related adjustments in long-running agents documentation.

Author: threepointone

src/content/docs/agents/api-reference/agents-api.mdx

@@ -91,6 +91,9 @@ flowchart TD
 | **Protocol messages** | `shouldSendProtocolMessages()`, `isConnectionProtocolEnabled()`                      | [Protocol messages](/agents/api-reference/protocol-messages/)       |
 | **Context**           | `getCurrentAgent()`                                                                  | [getCurrentAgent()](/agents/api-reference/get-current-agent/)       |
 | **Observability**     | `subscribe()`, diagnostics channels, Tail Workers                                    | [Observability](/agents/api-reference/observability/)               |
+| **Sub-agents**        | `subAgent()`, `abortSubAgent()`, `deleteSubAgent()`                                  | [Sub-agents](/agents/api-reference/sub-agents/)                    |
+| **Sessions**          | `Session.create()`, context blocks, compaction, search                               | [Sessions](/agents/api-reference/sessions/)                        |
+| **Think**             | `Think` base class, workspace tools, lifecycle hooks, extensions                     | [Think](/agents/api-reference/think/)                              |
 
 ## SQL API
 

src/content/docs/agents/api-reference/chat-agents.mdx

@@ -1245,6 +1245,12 @@ export class ChatAgent extends AIChatAgent {
 
 ### Subagent delegation
 
+:::note
+
+This section covers **in-process** subagents using the AI SDK's `ToolLoopAgent`. For **Durable Object sub-agents** with their own isolated storage and typed RPC, refer to [Sub-agents](/agents/api-reference/sub-agents/). For streaming full LLM turns through a child agent, refer to [Think: Sub-agent RPC](/agents/api-reference/think/#sub-agent-rpc-and-programmatic-turns).
+
+:::
+
 Tools can delegate work to focused sub-calls with their own context. Use [`ToolLoopAgent`](https://ai-sdk.dev/docs/reference/ai-sdk-core/tool-loop-agent) to define a reusable agent, then call it from a tool's `execute`:
 
 <TypeScriptExample>

src/content/docs/agents/api-reference/durable-execution.mdx

@@ -283,7 +283,7 @@ Key points:
 
 ### Chat recovery
 
-`AIChatAgent` builds on fibers for LLM streaming recovery. When `unstable_chatRecovery` is enabled, each chat turn is wrapped in a fiber automatically. The framework handles the internal recovery path and exposes `onChatRecovery` for provider-specific strategies. Refer to [Long-running agents: Recovering interrupted LLM streams](/agents/concepts/long-running-agents/#recovering-interrupted-llm-streams) for details.
+`AIChatAgent` builds on fibers for LLM streaming recovery. When `chatRecovery` is enabled, each chat turn is wrapped in a fiber automatically. The framework handles the internal recovery path and exposes `onChatRecovery` for provider-specific strategies. Refer to [Long-running agents: Recovering interrupted LLM streams](/agents/concepts/long-running-agents/#recovering-interrupted-llm-streams) for details.
 
 ## Concurrent fibers
 
@@ -344,4 +344,4 @@ Run an async function while keeping the DO alive. Heartbeat starts before `fn` a
 - [Long-running agents](/agents/concepts/long-running-agents/) — how fibers compose with schedules, plans, and async operations
 - [Schedule tasks](/agents/api-reference/schedule-tasks/) — `keepAlive` details and the alarm system
 - [Workflows](/agents/concepts/workflows/) — durable multi-step execution outside the agent
-- [Chat agents](/agents/api-reference/chat-agents/) — `unstable_chatRecovery` and `onChatRecovery`
+- [Chat agents](/agents/api-reference/chat-agents/) — `chatRecovery` and `onChatRecovery`