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/client/SideChatView.tsx

@@ -58,6 +58,7 @@ import {
   sideThreadRows,
   threadHasCompletedTurn,
   threadTrailingPending,
+  type SidechatLiveEvent,
   type SidechatThreadInfo,
 } from '../sidechat-core.ts'
 import {
@@ -112,6 +113,8 @@ const inFlightStarts = new Set<string>()
  * the afterSeq delta and never re-download what they already hold). */
 interface ThreadCache {
   entries: SidebarHistoryEntry[]
+  /** The CURRENT attempt's live deltas (replaced every pull, never merged). */
+  live: SidechatLiveEvent[]
 }
 
 /** Row-render labels (locale-dependent, memoized once per mount). */
@@ -383,7 +386,7 @@ export function SideChatView(props: {
   const [info, setInfo] = useState<SidechatThreadInfo | null>(null)
   const [menuOpen, setMenuOpen] = useState(false)
 
-  const cacheRef = useRef<ThreadCache>({ entries: [] })
+  const cacheRef = useRef<ThreadCache>({ entries: [], live: [] })
   // The previous poll's rows (see the mapping's reuse pass below).
   const prevRowsRef = useRef<SidechatTranscriptRow[]>([])
   const controllerRef = useRef<AbortController | null>(null)
@@ -457,14 +460,18 @@ export function SideChatView(props: {
     try {
       const cache = cacheRef.current
       const afterSeq = cache.entries.at(-1)?.event.seq
-      const { events } = await api.sidechatEvents(childId, afterSeq, controller.signal)
+      const { events, live } = await api.sidechatEvents(childId, afterSeq, controller.signal)
+      // The live set is the CURRENT attempt, so it replaces whatever the
+      // previous pull held; a settled step drops out of the host buffer and
+      // its durable assistant/message takes over in the mapping.
+      cache.live = live ?? []
       if (events.length > 0) {
         // Wire events arrive as parsed JSON; the mirror narrows data to the
         // record the mapping reads.
         const incoming = events.map(event => ({ event: event as SidebarSessionEvent }))
         cache.entries = mergeBySeq(cache.entries, incoming)
-        setRevision(value => value + 1)
       }
+      setRevision(value => value + 1)
     } catch {
       // Aborted by a newer pull or a wire failure: keep the last rows.
     }
@@ -482,7 +489,7 @@ export function SideChatView(props: {
   // Reset the transcript cache whenever the binding changes, then focus
   // the composer — it owns the first message of a fresh thread.
   useEffect(() => {
-    cacheRef.current = { entries: [] }
+    cacheRef.current = { entries: [], live: [] }
     prevRowsRef.current = []
     controllerRef.current?.abort()
     setError(null)
@@ -532,8 +539,24 @@ export function SideChatView(props: {
   // The previous poll's rows ride into the mapping so unchanged rows keep
   // their object identity (see reuseRows): the 2s poll re-renders only the
   // changed tail instead of re-parsing markdown for the whole transcript.
+  // The live deltas are appended AFTER the durable events (they are the
+  // in-flight tail), and a delta whose step already settled durably is
+  // dropped: the durable message is authoritative and would otherwise render
+  // twice.
   const rows = useMemo(() => {
-    const next = threadId === undefined ? [] : transcriptRows(cacheRef.current.entries, prevRowsRef.current)
+    const cache = cacheRef.current
+    const settled = new Set(
+      cache.entries
+        .filter(entry => entry.event.type === 'assistant/message')
+        .map(entry => {
+          const data = entry.event.data as { turn?: unknown; step?: unknown }
+          return `${String(data.turn)}:${String(data.step)}`
+        }),
+    )
+    const live = cache.live
+      .filter(event => !settled.has(`${String(event.data.turn)}:${String(event.data.step)}`))
+      .map(event => ({ event: event as SidebarSessionEvent }))
+    const next = threadId === undefined ? [] : transcriptRows([...cache.entries, ...live], prevRowsRef.current)
     prevRowsRef.current = next
     return next
   },