New chat screen: the "Start a chat" center gains channel-intro-style
preset cards. Default agent shows who will handle the chat with a
picker to swap in another managed agent or a whole team (team personas
are created in the chat like template agents, first one becomes the
default). Directory shows the selected project's path (or free-chat
empty state) and opens the project picker. Invite searches the user
directory and pre-adds people as members on create, merged with the
@mention flow.
Multi-agent activity: the chat transcript and working indicators only
read the default agent, so a second agent's turns rendered nothing.
The transcript now merges every active managed agent's items by
timestamp, live turn ids come from the by-channel store (which now
carries turnIds), and the composer stop button cancels every agent
working in the chat.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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