[TheGreatAxios]

Summary

• use-workbench-feed now loads a room's messages and reply-thread rows by listing and reading @corbits/mailbox threads instead of @corbits/chat's message/thread routes, and its live-update subscription follows the mailbox's own SSE stream (/me/inbox/events) instead of folding chat.message stream events straight into the cache.
• Pins and reactions are removed from chat-ui entirely (client-only; no server-side change) — the feed they hung off no longer exists in this shape.
• Every mailbox read stays behind one adapter, packages/chat-ui/src/mailbox-timeline.ts: it makes its own fetch() calls and validates responses with local arktype schemas rather than importing @corbits/inbox/client, whose value-level re-exports pull @corbits/mailbox's server-only migrations.ts (a node:crypto user) into the browser bundle and break the Vite build. Per the owner's ruling that @corbits/mailbox is a temporary, shrinking surface over Interchange's native mailbox, no other file in this package touches the mailbox response shape, so swapping to the native thread shape later is a one-file change.
• Deletes the now-uncalled @corbits/chat thread-messages read route (GET /workbenches/:id/threads/:threadId/messages) and the chat-ui pinned-strip/reaction-chip UI and wire types it fed.

Known regressions, follow-up needed

• Forking is disconnected: forking a message into a depth-2 sub-thread still targets the old @corbits/chat thread store, whose ids have no relationship to the new mailbox-derived thread ids. The fork/sub-thread affordance needs to be ported to mailbox threads before it works again.
• Block-kind message parts don't round-trip: approve/poll/form/connect-github/connect-service cards can't survive a reload — a mail body is plain text, so threadMessagesToTimeline only ever produces text parts. A reloaded page loses any persisted block content.
• Live updates are coarse: the mailbox SSE payload carries only a message id/op, not a thread root, so the subscription refreshes the whole feed on any event rather than re-reading just the affected thread.

Deleted tests

• packages/chat-ui/src/pins-status.test.ts
• packages/chat-ui/test/reactions-and-pins.test.tsx
• packages/chat-ui/test/connect-card-settings-stream.test.tsx — block-kind message parts can't round-trip through a mail body; no replacement mechanism exists yet (see regression above)
• packages/chat-ui/test/stream-apply.test.ts — applyStreamMessage/applyStreamReaction/applyStreamPin describe blocks (kept ensureReplyThreadRow)
• packages/chat/test/routes.test.ts — "threads — root feed vs reply membership (4a)" (exercised the now-deleted thread-messages route)
• packages/chat-ui/test/chat-workspace.test.tsx:
  • "a stale thread reference self-heals instead of dead-ending" — the GET .../threads/:id/messages 404-recovery path it tested never existed in the real feed even before this change
  • "forking a message inside a thread opens a sub-thread..." and "the threads menu indents sub-threads..." — see the forking regression above
• packages/chat-ui/test/zero-refetch-on-stream-event.test.tsx — "a streamed chat.message renders with no fetch call beyond mount-hydration" (the zero-refetch guarantee no longer holds once reads come from the mailbox)
• packages/chat-ui/test/message-hover-toolbar.test.tsx — reaction-picker and pin-menu-entry tests

Test plan

• bun run typecheck
• bun run lint
• bun run check:structural
• bun --cwd packages/chat-ui test (852 pass)
• bun --cwd apps/web test (799 pass, includes a real vite build)
• bun --cwd packages/chat test (808 pass, 66 skip — DB-gated)

[linear-code]

CL-8174