refactor(client): carve outward interfaces for the session, workspace, layout, slash, and conversation services

Feature packages now reach these domains through interface types only:
ISession/SessionFace (identity + prompt/cancel/loadOlder + the useSession
snapshot source), ISessions, IWorkspaces, ILayout, IConversation, and the
existing SlashServiceContract now actually mounted on Context.slash. The
concrete services implement their face; wire-pump and assembly entry
points stay on the classes. The provide-channel materialization and
current-projection logic moves into SessionProvideChannel so the
production service and the client test runtime share one implementation.
The workspaces service consumes sessions through the narrow SessionsPort.

packages/client/runtime/src/client/contract/sessions.ts

@@ -0,0 +1,64 @@
+/**
+ * The outward sessions-service face — what `ctx.sessions` exposes to feature
+ * packages and the renderer host, and therefore exactly what the test
+ * runtime's sessions double must implement. Wire-pump entry points
+ * (handleMuxEnvelope/handleConnected/refresh) and runtime internals stay on
+ * the concrete class; cross-domain consumers keep the narrower
+ * [SessionsPort](./sessions-port.ts). Widening this interface is the
+ * explicit act of widening what features may do to the sessions domain.
+ */
+import type { Context } from 'cordis'
+import type { SessionId } from '@deepseek-ai/dsh-client-connection/client'
+import type { HostObservable, SessionMaybeProvideInfo } from '@deepseek-ai/dsh-client-ui-slots'
+import type {
+  SessionBinding, SessionListState, SessionProvideDescriptor,
+} from '../sessions/service.ts'
+import type { SessionFace } from './session.ts'
+import type { ObservableSnapshot } from './store.ts'
+
+/** The sessions-service face injected as `ctx.sessions`. */
+export interface ISessions {
+  /** The useSessions standard feed (list rows + current selection; read face — writes stay inside the domain). */
+  readonly list: ObservableSnapshot<SessionListState>
+  /** Atomic current-session provide projection (the renderer host's `sessions.provideInfo` feed). */
+  readonly currentProvideInfo: HostObservable<SessionMaybeProvideInfo>
+  /**
+   * Select a session as current.
+   * @param id - session id (must exist in the list; unknown ids fail loud).
+   */
+  open(id: SessionId): void
+  /** Clear the current selection into the no-session view state. */
+  clear(): void
+  /**
+   * Register a per-session standard-props provider (hooks become `use<Name>`
+   * selector hooks on the render side; props spread verbatim).
+   * @param descriptor - static member roster plus per-session resolver.
+   * @returns disposer removing the provider.
+   */
+  provide(descriptor: SessionProvideDescriptor): () => void
+  /**
+   * Resolve an Agent-scoped context view (use-and-discard).
+   * @param id - session id.
+   * @returns scoped ctx, or undefined for a session neither listed nor already scoped.
+   */
+  scope(id: SessionId): Context | undefined
+  /**
+   * Read the Agent scope tag off a context (service-method seam: fetch
+   * bundles must reach scope resolution through ctx.sessions).
+   * @param ctx - any client context.
+   * @returns the session id, or undefined on root contexts.
+   */
+  scopeOf(ctx: Context): SessionId | undefined
+  /**
+   * Resolve the session face behind an Agent-scoped context.
+   * @param ctx - an Agent-scoped context.
+   * @returns the session face, or undefined when the ctx is untagged or its scope was pruned.
+   */
+  sessionOf(ctx: Context): SessionFace | undefined
+  /**
+   * Resolve the stable session binding (scope-addressed assembly feed).
+   * @param id - session id.
+   * @returns binding, or undefined for a session neither listed nor already scoped.
+   */
+  binding(id: SessionId): SessionBinding | undefined
+}