feat(sidechat): 侧边对话——每对话一 Tab（Codex 对齐）+ 完整上下文继承 + 保存为新会话 (#286)

三轮实测驱动迭代：种子信封透传（surfaceOp 校验）、每线程一 Tab 交互重做完、四连修（fork 丢 this / 自绘 history+save 图标 / collectOwnEvents 大页回源修重开丢工具行 / descriptor 入种子修目录 corrupt 行 + 前缀过滤保拓扑零噪音）。739 单测 + 挂载冒烟 12/12 全绿。

src/client/builtins/tabs.tsx

@@ -1,13 +1,13 @@
 /**
- * The 6 built-in tab descriptors: the plugin registers its own pages
- * (editor / git / terminal / browser / subagent / diff) through
+ * The 7 built-in tab descriptors: the plugin registers its own pages
+ * (editor / git / subagent / sidechat / terminal / browser / diff) through
  * the same {@link BetterSidebarService} external plugins use — eating its
  * own dogfood. The terminal descriptor owns its quota (`TERMINAL_LIMIT`)
  * and mints `terminal:<uuid>` ids through `createTab`; the browser mints
  * `browser:<n>` the same way (no quota). The editor IS the files window
  * (the old standalone explorer merged into it).
  */
-import { IconBranchOutline16, IconCodeOutline16, IconFolderOpen16, IconPanelLeftOutline16, IconThinkOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
+import { IconBranchOutline16, IconCodeOutline16, IconFolderOpen16, IconNewChatOutline16, IconPanelLeftOutline16, IconThinkOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { Context } from '../../context-types.ts'
 import { allLeaves, isAgentTabId, type SidebarState } from '../state.ts'
 import { t } from '../locales.ts'
@@ -17,6 +17,8 @@ import { lazyChunkComponent } from '../lazy-chunk.tsx'
 import { GitView } from '../GitView.tsx'
 import { DiffTab } from '../DiffTab.tsx'
 import { SubagentView } from '../SubagentView.tsx'
+import { consumeSidechatSeed, SideChatView, sidechatThreadIdOf } from '../SideChatView.tsx'
+import { api } from '../api.ts'
 import { BrowserView } from '../BrowserView.tsx'
 import { IconTerminalOutline16, IconDiffOutline16, IconGlobeOutline16 } from '../icons.tsx'
 import { TERMINAL_FONT_SIZE_MAX, TERMINAL_FONT_SIZE_MIN } from '../../prefs-shared.ts'
@@ -165,6 +167,51 @@ export function builtinTabs(ctx: Context, options: BuiltinTabOptions = {}): read
         />
       ),
     },
+    {
+      id: 'sidechat',
+      title: () => t('sideChat'),
+      icon: (size: number) => <IconNewChatOutline16 size={size} />,
+      order: 35,
+      // Codex-style: EVERY side conversation is its own tab. A plain open
+      // mints a fresh tab flagged `autoCreate` (the view creates the EMPTY
+      // thread on mount); a thread switch from the header menu parks the
+      // target id for a deterministic `sidechat:<threadId>` reattach tab.
+      createTab: () => {
+        const threadId = consumeSidechatSeed()
+        if (threadId !== undefined) {
+          return {
+            tab: {
+              id: `sidechat:${threadId}`,
+              type: 'sidechat',
+              title: t('sideChat'),
+              meta: { threadId },
+            },
+          }
+        }
+        return {
+          tab: {
+            id: `sidechat:new-${crypto.randomUUID()}`,
+            type: 'sidechat',
+            title: t('sideChatUntitled'),
+            meta: { autoCreate: true },
+          },
+        }
+      },
+      // One tab per thread: an already-open thread focuses instead of
+      // duplicating; unbound fresh tabs never dedupe (each mints its own).
+      dedupeKey: (tab) => sidechatThreadIdOf(tab),
+      // Closing the tab releases the thread's live agent; the session and
+      // its history stay persisted (reopen from any thread's header menu).
+      onClose: (tab) => {
+        const threadId = sidechatThreadIdOf(tab)
+        if (threadId !== undefined) {
+          void api.sidechatDispose(threadId).catch(() => {})
+        }
+      },
+      component: ({ ctx, scope, tab, visible }) => (
+        <SideChatView ctx={ctx} scope={scope} tab={tab} visible={visible} />
+      ),
+    },
     {
       id: 'terminal',
       title: () => t('terminal'),