fix(web): a new chat starts its own thread instead of appending to Myra's last one (CL-8616)

[TheGreatAxios]

Summary

A chat's route id used to be the agent's definition asset id, so listChats/readChat in apps/web/src/chat/threads-api.ts grouped every mail turn to that agent's address set into one conversation. Starting a chat from /chats/new sent a fresh, non-reply message, but the chats list still filed it under the agent's single existing entry, so a new question showed up appended under whatever conversation was already there.

The chat id is now the thread's root turn id (Sent:<uid> of the person's opening send), found by walking each turn's In-Reply-To/References back to a turn with no known parent (threadRootIds). listChats groups by that root instead of by agent, and readChat resolves a chat's rows and its agent from the thread rather than from an agent lookup. startChat returns the new root id from the send's own accepted response instead of the agent's asset id.

chat-thread-page.tsx's agent avatar now keys off chat.agent.id instead of the (now thread-scoped) chat.id. Updated docs/chat-mail-threading.md to describe the new keying.

Testing

• bun run check (typecheck, lint, fmt, test) passes from the worktree root.
• Updated apps/web/src/chat/threads-api.test.ts fixture for the new MailTurn.parentId field.

[linear-code]

CL-8616