Scope restored chat selections per chat

[rschmukler]

Include chat-scoped model, variant, and trust updates during open, resume, and import, and return an atomic selection snapshot from chat/open.

Summary

• Scope model, variant, and trust restoration from chat/open, /resume, and /import to the affected chat using chatId.
• Return an atomic selection snapshot from chat/open containing the effective model, agent, variant, available variants, and trust state.
• Keep scoped config/updated notifications for backward compatibility.
• Correct the chat/open response field from found? to the documented found.
• Add unit and end-to-end regression coverage for selection restoration.

Why

Previously, restoring chat A emitted session-wide config/updated notifications. Clients could therefore apply chat A’s model, variant, or trust state to chat B and to the defaults inherited by future chats.

Scoping these updates isolates each chat’s selection state. Including the complete selection in the chat/open response also lets clients hydrate a restored chat atomically rather than reconstructing its state from multiple independently ordered notifications.

• I added a entry in changelog under unreleased section.
• This is not an AI slop.

[ericdallo]

Server looks good, thanks!