Why: T4 workspaces are declared by plugins via provides.workspaces (Core PluginWorkspace is the authoritative type); the host must surface the merged overlay for the frontend to render in OverlayHost.
Evidence: go test green - plugins suite new TestWorkspacesOverlay (manifest + workspaces/main.json loads, ListWorkspaces returns the declaration with shortcut and body); full internal sweep unchanged.
Impact: plugins host loads workspaces/*.json assets and exposes ListWorkspaces; server adds GET /api/workspaces passthrough; no behavior change without the manifest key.
Refs: docs/proposals/2026-09-30-ui-inbox-command-workspace.md T4; task card feature/workspaces-r1
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