Add session/conversation naming to SessionUpdate

[akosyakov]

Problem

ACP clients that want to display a human-readable name for a conversation have no way to get one from the protocol. The SessionUpdate message has no name, title, or summary field.

This forces clients to implement their own naming outside ACP — typically by making a separate LLM call with the conversation content and a "generate a title" prompt. Zed does exactly this (client-side summarize() call to a dedicated model). This means every ACP client needs its own LLM access and its own title-generation logic, which is redundant work.

Proposal

Add an optional title (or name / summary) field to SessionUpdate so that the agent can provide a human-readable conversation title as part of the normal session stream.

The agent already has full context of the conversation and LLM access — it's the natural place to generate a title. Clients would simply read the field and display it.

Suggested shape

interface SessionUpdate {
  // ... existing fields ...
  
  /** Optional human-readable title for the conversation, generated by the agent. */
  title?: string;
}

Behavior

• The field is optional. Agents that don't support naming omit it.
• The agent may send the title at any point during the session (e.g., after the first response).
• Clients should treat the latest title value as the current name, replacing any previous value.
• If the agent never sends a title, the client can fall back to its own naming strategy or show a default.

Motivation

• Clients shouldn't need independent LLM access just to name conversations.
• The agent has the conversation context already — duplicating it for a title call is wasteful.
• A protocol-level field makes naming consistent across all ACP clients instead of each reimplementing it differently.

[morozow]

@akosyakov Add the full session context type to evaluate its own technical context before an additional field is added please.

I cannot recognize any business case where the protocol itself should set it up. Can you help with a list of ACP requirements covered by the additional field?