feat(sdk): let a client send context with a user message

[matej21]

Problem

A client often knows something the user does not type: the page shown in a preview, a selected element, the editor state. The agent needs this to understand messages like "make this heading bigger".

@roj-ai/client-react already has a place for it. useSessionStore has messageContext and setMessageContext. But sendMessage never reads the value, and user-chat.sendMessage has no field that could carry it. The context is computed and then dropped. A production consumer shipped a "select an element and ask the agent" feature on top of this store, and the agent never received the selection.

Change

• user-chat.sendMessage takes an optional context: string.
• The user_chat_message_received event and UserChatMessage get an optional context. The field is omitted when the client sends none, so events and state for plain messages do not change.
• The LLM receives the context after the message text:

  [User]: Make this heading bigger
  <user_context>
  Page: /about
  </user_context>
  

• content stays what the user typed. A UI that renders content does not show the context.
• The hard limit (MESSAGE_MAX_TOKENS) counts content and context together. Soft truncation still applies to content only.
• client-react: sendMessage sends messageContext as context.

Compatibility

• The field is optional, so existing callers do not change.
• An older agent server strips the unknown context key (z.object), so a new client works with an old server. The agent then does not see the context, as before.

Tests

• New tests in user-chat.integration.test.ts:
  • A message with context: the LLM request contains the <user_context> block. The chat history keeps the context apart from content.
  • A message without context: the LLM request has no block, and the event has no context key.
• bun run test: 1606 pass, 77 skip, 0 fail.
• bun run ts:build, bun run lint and tsc -p packages/sdk/tsconfig.test.json are clean.
• Demo e2e (snapshot replay): 2 pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_012fX9ohNay3XuPYkKGdGZ3i