feat(web): add workspace-aware session flow

packages/client/ui-workspace/README.md

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+# @deepseek-ai/dsh-client-ui-workspace
+
+Shared Workspace picker plugin. `WorkspacePicker` is registered into the sidebar's `sidebar.workspace` slot and the page-local Session Intent hero's `conversation.empty.workspace` slot, so both surfaces use the same menu and creation modals.
+
+The picker lists real Host Workspace entities through the global `useWorkspaces` hook. Selecting a Workspace invokes the slot owner's `onPick` callback to retarget the frontend Session object; the existing-folder and create-new actions first create a real Workspace through the object layer, then select it. Create-new disables names already present in that list, while the Host remains authoritative for concurrent or non-UI callers. The runtime Session and Workspace services own materialization.
+
+Both target slots are declared by other plugins, so `apply` registers through declaration-aware deferral and re-registers after a declaring slot is restored.
+
+## Model Experience
+
+None, as the picker is browser chrome; nothing here reaches a model request.
+
+#### KV Cache effect
+
+None; this package neither assembles nor sends a provider request.
+
+## Known Limitations and Deferred Work
+
+- **No Workspace rename/delete controls** — the picker supports selection and creation only.
+- **Existing-folder entry is manual path input only** — Host creation failures are shown in the modal.