fix(web): the workbench's Artifacts panel refreshes when an agent saves an artifact (CL-8604)

[TheGreatAxios]

Summary

• The workbench page's mailbox SSE subscription already invalidated the workbench timeline/participants and pending-approvals queries on every inbound mailbox event; it now also invalidates tenantKeys.artifacts(workbenchTenantId), which covers both the Artifacts panel and (via the nested artifactCounts key) the library page's kind counts.
• No new fetches or polling added; the handler cannot distinguish agent messages from the person's own, so it invalidates on every inbound mailbox event, same as the existing invalidations in that handler.

Testing

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

[linear-code]

CL-8604