feat(dsh): adapt to DSH 0.1.5-alpha.1

Move the plugin onto the DSH 0.1.5-alpha.1 baseline (peer floor
^0.1.5-alpha.1, CI pinned to the alpha dist-tag) and fix the two host
contract changes that alpha line introduced.

Baseline:
- all @deepseek-ai/dsh-* peer/dev deps -> 0.1.5-alpha.1; optional peer
  @deepseek-ai/dsh-client-ui-sidebar-right (declared, unused until the
  native-surface phase); devDep @deepseek-ai/dsh-agent-loop (inbox fold)
- dsh.plugin.json engines >=0.1.5-alpha.1; version 0.19.0-alpha.0;
  SIDEBAR_SERVICE_VERSION follows

Live assistant stream (assistant/chunk is gone):
- 0.1.5 publishes in-flight model deltas as process-local
  agent/assistant-stream frames instead of durable assistant/chunk events
  and embeds the settled stream in assistant/message / assistant/attempt.
  src/assistant-live.ts folds the frames into a bounded per-session buffer;
  sidechat.events serves the current attempt as `live` rows (replaced every
  poll, never a delta) and the transcript renders them; the inherited
  in-progress snapshot reads message content / attempt stream / live frames.

Cold session reads (persistence.inspect is gone):
- 0.1.5 reads a stored session through an explicit read handle
  (open(id,'read') -> handle.read() -> close()). src/session-store.ts owns
  that protocol; every cold read (sidechat transcript, cold-resume preset,
  session.cwd, changes replay) goes through it.

Dead code and docs:
- drop the remote.session.openWorkspacePath interception: 0.1.5's chat opens
  files through ctx.sidebarRight.openResource instead, so the wrapper had no
  caller and would hijack the host's "open in app" gesture; the
  interceptOpenPath pref, its settings row and its locale keys go with it
- AGENTS.md §3 + external-plugin-guide.md describe the new baseline
- vitest setup bridges Node's localStorage accessor to jsdom's store (the
  pre-existing 26 failures on Node >= 22)
- desktop-layout e2e seeds through host RPC and types into the Lexical
  composer (0.1.5 replaced the textarea) instead of driving the directory
  picker

Verified: pnpm typecheck / lint / build / test (124 files, 1327 tests) /
check:consumer-types, plus pnpm test:mount against a real
@deepseek-ai/dsh@0.1.5-alpha.1 host (18/18).

src/context-types.ts

@@ -296,14 +296,34 @@ export interface SidebarSessionTitleService {
   rename(session: unknown, title: string): { title: string; eventSeq: number }
 }
 
-/** The host session-persistence face (mirror of the sessionPersistence
- *  service): detached inspection of a persisted session, used to compose the
- *  recorded preset when a Side Chat thread cold-resumes. */
+/**
+ * The host session-persistence face (mirror of the `sessionPersistence`
+ * service): durable, handle-addressed session storage.
+ *
+ * DSH 0.1.5 replaced the detached `inspect(id)` call with an explicit read
+ * handle: `open(id, 'read')` never takes write ownership and works while
+ * another process owns the session, `handle.read()` returns one contiguous
+ * slice of the log, and `close()` releases it. Every cold read in this plugin
+ * goes through {@link readPersistedSession} so the handle is always closed.
+ */
 export interface SidebarSessionPersistenceService {
-  inspect(sessionId: string): Promise<{
-    meta: { cwd?: string; agentPreset?: string }
-    events: readonly SidebarSessionEvent[]
-  }>
+  open(sessionId: string, access: 'read' | 'write'): Promise<SidebarSessionHandle>
+}
+
+/** One open channel onto a stored session (the fields this plugin reads). */
+export interface SidebarSessionHandle {
+  /** Immutable stored header (cwd / agentPreset live here). */
+  readonly header: { cwd?: string; agentPreset?: string } & Record<string, unknown>
+  /** Exact fork-inherited prefix length stored with the log. */
+  readonly inheritedEventCount?: number
+  /**
+   * Read a slice of the valid contiguous log.
+   * @param offset - first logical seq to include (defaults to 0).
+   * @param length - maximum events (defaults to the rest of the log).
+   */
+  read(offset?: number, length?: number): Promise<{ events: readonly SidebarSessionEvent[] }>
+  /** Release the handle (idempotent). */
+  close(): Promise<void>
 }
 
 /** The client session list snapshot the sidebar subscribes to. */
@@ -408,30 +428,6 @@ export interface SidebarConversation {
   }
 }
 
-/**
- * The client `remote.session` namespace face (mirror of the gateway client's
- * RemoteNamespaceService for the session-controller contribution on alpha
- * hosts). The chat's file-open funnel is `openWorkspacePath`: the caller
- * resolves the path against the session cwd, and the host hands it to the
- * OS's default application. Namespace methods are accessor properties (see
- * `client/openpath-intercept.ts` for how the interception shadows them).
- */
-export interface SidebarRemoteSessionService {
-  /**
-   * Open an absolute path with the Host operating system's default
-   * application. Resolves with the typert `RemoteResult` envelope
-   * (`{ ok: true, value: { opened } }` / `{ ok: false, error }`), like every
-   * remote method — callers branch on `result.ok`.
-   */
-  openWorkspacePath(
-    request: { path: string },
-    signal?: AbortSignal,
-  ): Promise<
-    | { readonly ok: true; readonly value: { opened: boolean } }
-    | { readonly ok: false; readonly error: { readonly code: string; readonly message: string; readonly details: object } }
-  >
-}
-
 /**
  * The invariant service face (mirror of @deepseek-ai/dsh-invariants'
  * InvariantRegistry). The upstream augmentation does not reach this Context
@@ -564,6 +560,13 @@ export interface SidebarContextShape {
    * LIVE Session instance that appended it.
    */
   on(event: string, listener: (session: unknown, event: SidebarSessionEvent) => void): () => void
+  /**
+   * The agent's process-local assistant stream (DSH 0.1.5+): one payload per
+   * `start` / `chunk` / `end` frame, carrying the emitting agent and the
+   * frame. These frames are NOT session events — see
+   * {@link ./assistant-live.ts} for why the plugin needs them.
+   */
+  on(event: 'agent/assistant-stream', listener: (payload: { agent?: unknown; frame?: unknown }) => void): () => void
 }
 
 /**