[TheGreatAxios]

Summary

• The composer's send path now posts straight to @corbits/mailbox's POST /me/inbox/send ({ to, body, inReplyTo? }) instead of the legacy @corbits/chat message route — matching the reads, which already moved onto the mailbox in CL-8174.
• to is every other participant's mailbox address (Workbench.participants, minus the signed-in sender by principal-id local part). inReplyTo is the RFC Message-ID of the thread root (or the specific message being replied to when starting a fresh thread) — resolved from the loaded feed's own messages, since openThreadId/pendingParentMessageId are mailbox uids, not wire ids.
• Deleted the legacy sendMessage client, its SendMessageOptions/MessageInviteInput types, and the now-unused SentMessage schema; added sendInboxMessage in their place.
• instant-agent-create.ts's opening-message send (new-workbench picker) moved onto the same call: it now invites each selected agent first (inviteAgent, which returns the agent's own address) and sends the opening message to those addresses.
• MessageItem gained an optional messageId (the RFC Message-ID), populated by mailbox-timeline.ts for every mail-derived row.

Known gaps (left for follow-up, not moved off the old route)

• Attachments: the mailbox send wire has no attachment field, so a composer submit with a file attachment now sends only its text — attachments are silently dropped rather than shimmed. No mail-attachment API exists to move this onto.
• Mention pre-invite (invite on MessageSendOptions): the chat route resolved "Bring in…" mention picks server-side before the send; the mailbox route has no such concept, so that fan-out is gone from the composer's own send path. Inviting an agent explicitly (Settings → Agents / /invite) still works.
• instant-agent-create.ts's opening message is only sent when at least one agent was selected — mail requires a non-empty to, so an opening message typed with zero agents picked is no longer attempted (previously it posted into an ownerless empty room).

Test plan

• bun run typecheck — clean outside packages/chat-ui/**/*.test.* (another agent is deleting/rewriting those in parallel per the dispatch instructions; two pre-existing test files there now fail to compile against the deleted sendMessage export and the new useOptimisticSends args)
• bun run lint
• bun run fmt
• bun test in apps/web — 768 pass, 3 pre-existing unrelated Tailwind-CSS failures
• bun test in packages/chat-ui — failures are confined to packages/chat-ui/**/*.test.* files (out of scope per dispatch instructions)

[linear-code]

CL-8175