feat(web): workbench room is the child tenant's mail thread; legacy chat client deleted (CL-8435)

[TheGreatAxios]

What

/w/:tenantId is now a room over the child tenant's mailbox, and the client for the deleted /chat/workbenches/* routes is gone. Rebased onto current main (#870–#874).

• pages/workbench-room-page.tsx — header (tenant name), participants (people from GET /api/tenants/:id/principals + agents from the child's live deployment run addresses), timeline built from the room mailbox's Sent + INBOX threaded by In-Reply-To/References, sub-thread replies in a side panel, composer addressed to every agent in the room. TanStack Query throughout (roomKeys in chat-path.ts); the only effect is the mailbox SSE subscription, which invalidates.
• chat/threads-api.ts — room additions: listRoomParticipants, readRoom, sendToRoom. sendToRoom is the single send seam and now uses main's mailbox send route (POST /api/tenants/<childId>/mailbox/me/inbox/send with to, body, optional inReplyTo), so the hub triggers the agents and keeps the Sent copy — no localStorage anywhere.
• workbench-create.ts + pages/new-workbench-picker.tsx — create child tenant (POST /api/tenants {name,slug,parentId}) → publish the tool-package registry into it → resolve its own offering (resolveExistingOffering(childId), which inherits) → deploy Myra into the child (deployMyraSource + stock POST /workflows/deployments) → send the opening message through the child's mailbox → navigate in the mutation's onSuccess.

Why deploy rather than invite: verified on a live hub that the stock member invite only accepts a real user's email, and a child tenant's deploy route rejects the parent's inherited asset (404 "Workflow asset not found") while its resolved models/offerings do inherit.

Deleted

chat/api.ts and everything that only served it: chat-workspace, timeline, composer, mentions, typing-indicator, use-workbench-feed, use-workbench-stream, workbench-presence, streaming-reply, direct-workbench, shared-workbenches, default-agent-workbench, mailbox-timeline, thread-feed, use-optimistic-sends, use-thread-navigation, workbench-timeline-view, workbench-settings/, pr-thread-view, artifact-chip, chat/sidebar, profile-relations, myra-workbench, instant-agent-create, agent-chat-launch, agent-dm-launch, pages/chat-page.tsx, pages/workbench-timeline.tsx, workbench-timeline-merge, shell/tenant-resolution — plus their tests. Outdated tests were deleted, not adapted. grep -rn 'chat/workbenches|chat/api"' apps/web/src is empty.

Touched files owned by other tickets (minimum to unbreak)

• pages/insights-page.tsx — dropped the embedded WorkbenchTimelineRoute (its page is deleted) and switched to @/chat/workbench-tenants.
• shell/bench-activity.ts rewritten over stock listings, so agents-page.tsx, mission-control-page.tsx and library-page.tsx are untouched.
• command-palette-provider.tsx / command-palette-actions.ts — import path only; "talk to Myra" now lands on /chats/new.
• shell/context-menu/items.tsx — pin item removed (workbench settings surface deleted). shell/canvas-column.tsx — profile card keeps identity + mention; "Message" lands on /chats/new, shared-workbench channels empty (no stock read for another principal's rooms).
• agents-api.ts — listCapabilityInventory moved here from the deleted chat/api.ts, alongside feat(web): Agents and Skills pages read stock deployments; create agent deploys a workflow asset (CL-8433) #874's deploy hooks.

Gaps

• The new-workbench screen no longer offers an agent multi-select: only Myra's definition can be rendered and deployed into a room today (agent-deploy.ts deploys from a name + system prompt, which an existing bench definition does not expose). Adding more agents to a room is a follow-up on that pipeline.
• A room's agent is addressable once its deployment's run surfaces; an opening message sent before that is skipped rather than failing the create — the person sends it from the room.
• Read state (/me/inbox/:uid/read) is not wired; presence/typing/streaming dropped as ruled.

bun run check passes (typecheck, lint, fmt, 587 tests).

[linear-code]

CL-8435