Group chat members in a compact header popover

[pratikbuilds]

Groups the chat header’s member avatars and invite control into a compact Members button. The popover shows the roster with hover/focus highlighting, matching agent and human preview sizes, and a shared popover menu for Edit agent. Settings remains separate; the duplicate live-presence stack and header Remove action are removed.

Also adds a read-only owner projection to workbench responses so the owner appears when absent from the participant roster. This backend/API change remains in the draft; resolve whether to remove it to keep the change UI-only before marking ready.

Validation

• 77 focused tests passed across member UI, workspace composition, avatar roster, and backend owner suites.
• Chat UI typecheck and diff whitespace check passed.
• Repository pre-push checks passed (formatting, lint, typecheck, and test pipeline, including the web production build). Lint reported 10 warnings and no errors.
• A fresh browser walkthrough and a separate full bun run check have not been completed for this draft.

Refs #697.