* feat(sdk): let a client send context with a user message
`user-chat.sendMessage` takes an optional `context`: what the client knew
when the user sent the message (the page on screen, a selected element).
The LLM gets it in a `<user_context>` block after the message. The chat
history keeps it in a separate field, so a UI that renders `content`
does not show it. The hard token limit counts content and context together.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fX9ohNay3XuPYkKGdGZ3i
* fix(client-react): send the message context the store holds
`setMessageContext` stored the context, but `sendMessage` never read it,
so the agent did not receive it. Send it as the new `context` field.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fX9ohNay3XuPYkKGdGZ3i
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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>