docs: workspace and workbench terminology (CL-8568)

[TheGreatAxios]

Summary

• Aligns docs on the ruling: workspace is the top-level Interchange tenant, workbench (bench) is the child tenant /new creates.
• Removes "room" as a product term from docs and from user-facing web strings.

Changes

• docs/GLOSSARY.md: replaced the "Bench" entry with separate "Workspace" and "Workbench (bench)" entries; other entries updated to reference workspace where they described the top-level tenant.
• PRODUCT.md, DESIGN.md: replaced "room" with "workbench"/"conversation" in prose.
• apps/web/src/pages/new-workbench-picker.tsx, apps/web/src/pages/insights-page.tsx: user-facing toast/label text now says "workbench" instead of "room".

Code identifiers, file names, routes, and CSS classes that still say "room" were left untouched (out of scope): room-roster.ts/.test.ts, workbench-room-page.tsx, roomKeys, sendToRoom, listRoomParticipants, redeployRoomAgent, RoomParticipant/RoomMessage/RoomTurn, readRoom/readRoomFolder, RoomMessageSender, the room-* CSS class names, and WorkbenchRoomRoute/Room/RoomInfoColumn/RoomMessageRow components.

Testing

• bun run check (typecheck, lint, fmt, test) passes.

[linear-code]

CL-8568