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/sidechat-transcript.ts

@@ -13,9 +13,10 @@
  * shows only the thread's own conversation.
  *
  * Live streaming: `assistant/message` events only land when a step
- * completes, but `assistant/chunk` events stream token-level text and
- * reasoning deltas. The mapping accumulates both per block and supersedes
- * them with the assembled message once it lands (settled rows).
+ * completes, but the host's `assistant/live-chunk` rows stream token-level
+ * text and reasoning deltas (DSH 0.1.5 publishes them outside the session
+ * log — see assistant-live.ts). The mapping accumulates both per block and
+ * supersedes them with the assembled message once it lands (settled rows).
  */
 import type { DiffHunk, ReadBlockLine } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { SidebarHistoryEntry } from '../context-types.ts'
@@ -299,12 +300,13 @@ function resultCard(
 /**
  * Map a thread child's history rows onto compact transcript rows: the
  * inherited fork seed is cut at the last `session/end-seed`, context
- * injections map onto a collapsible injection row, `assistant/chunk`
+ * injections map onto a collapsible injection row, `assistant/live-chunk`
  * deltas accumulate into streaming rows per (turn, step, block) and are
  * superseded by the assembled `assistant/message`, and tool invocations
  * render one expandable line each (arguments, paired result text, failure
  * marker; a still-executing call is marked until its result lands).
- * @param entries - history rows (event + host-computed view) in seq order.
+ * @param entries - history rows (event + host-computed view) in seq order,
+ *   the live deltas of the in-flight attempt appended after the durable tail.
  * @returns display rows in log order.
  */
 export function transcriptRows(entries: readonly SidebarHistoryEntry[], prev?: readonly SidechatTranscriptRow[]): SidechatTranscriptRow[] {
@@ -374,7 +376,7 @@ export function transcriptRows(entries: readonly SidebarHistoryEntry[], prev?: r
         rows.push({ kind: 'user', seq: event.seq, text })
         break
       }
-      case 'assistant/chunk': {
+      case 'assistant/live-chunk': {
         const chunk = data.chunk as { type?: unknown; text?: unknown } | undefined
         if (chunk === null || typeof chunk !== 'object') break
         const kind = chunk.type === 'text-delta' ? 'assistant' : chunk.type === 'reasoning-delta' ? 'reasoning' : null