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.

[TheGreatAxios]

Closing: every file this draft touches has since been deleted from main, so there is no rebase path.

• packages/chat (including src/routes.ts, where the owner projection lives) was deleted in Delete packages/chat, commands, agent-runtime, turn-artifacts (CL-8175) #810 — chat is mail (CL-8175).
• packages/chat-ui (chat-workspace.tsx, workbench-presence.ts, api.ts and the tests here) was folded into apps/web in refactor(web)!: fold the Workbench-only UI packages into apps/web (CL-8319) #829 (CL-8319).
• feat(web): remove standalone chats; a workbench is the only conversation #968 then removed standalone chats: a workbench is the only conversation.

The duplicate presence stack this removes no longer exists, and a Members surface already lives in workbench settings (apps/web/src/chat/strings.ts). If the compact header Members popover is still wanted, it is a fresh change against apps/web/src/chat — worth a ticket that also settles whether the read-only owner projection is in scope, which this draft left open. Branch left in place for reference.