Expose peer-chat discovery and stable chat identifiers for agent-host sessions

## Problem

Agent Host sessions support multiple peer chats that share the same workspace and worktree. However, chats cannot discover or directly communicate with other chats that were created manually in the same session.

A chat may be able to send a message when it already has an opaque direct chat reference, but it cannot list sibling chats, retrieve its own reference, or target a sibling by its visible title. This prevents reliable multi-agent coordination inside a single session.

## Reproduction

1. Create an Agent Host session.
2. Create two or more peer chats manually in that session.
3. Ask one chat to list the other chats or send a message to a specific sibling by title.

## Actual behavior

The chat can access the shared workspace, but cannot enumerate sibling chats or send a message to a user-created peer chat by title.

## Expected behavior

Every chat in an Agent Host session should be able to discover and send a message to any other chat in that same session.

## Requested capability

Provide supported, session-scoped peer-chat communication:

- List peer chats in the current session with safe metadata such as title, status, and a stable target reference.
- Retrieve the current chat's own stable target reference.
- Send a message to any peer chat in the current session by title or stable reference.
- Surface incoming peer-chat messages clearly in the target chat.
- Optionally expose a **Copy Chat Reference** action in the UI.

The capability should be limited to chats within the same Agent Host session. Cross-session messaging should remain unavailable unless explicitly designed and authorized.

## Why it matters

This enables a coordinator chat to delegate work, check progress, provide follow-up direction, and consolidate results across chats that User created manually.

Without this, multi-chat sessions share files but cannot operate as a coordinated team. The user must manually relay context and instructions between chats, which defeats much of the value of running multiple agents in one session.

Start by tracing the existing Agent Host session and peer-chat communication entry points, especially how opaque direct chat references are created and used. Done means chats can list same-session peers, retrieve their own stable reference, target peers by title or reference, and clearly receive messages, while cross-session communication remains unavailable.

Written by the indexing model from the issue text.