feat!: 合并 Git 面板与文件追踪为统一「文件变动」tab（双视角 + 统一 diff 栈）

在 PR #471（file-trace）基础上的演进：

- 合并 tab：git tab（id 保留，兼容持久化布局）升级为「文件变动」双视角
  tab——Git 视角（原 GitView 全部能力：staged/unstaged/提交/历史/worktree·
  子仓库选择/右键菜单/确认弹窗）+ 本轮文件视角（原 file-trace：会话事件
  日志实时折叠的模型读写编辑记录，按文件分组、新增类型筛选 chips、相对
  时间）；file-trace tab 移除（未随任何版本发布）。
- 统一 diff 组件：新 src/client/diff/ 模块（rows.ts 引擎 + DiffRows/
  DiffFiles 渲染器 + highlight.ts 语法着色）——git diff 与会话操作 diff
  共用一套渲染：mod 改写配对（del-run/add-run 对半标蓝）、行内字符级
  高亮（修复了旧实现旧侧失配不高亮的问题，改为行对象身份键）、上下文
  hunk 折叠、git hunk 间未输出区间按行号范围生成不可展开折叠、语法着色
  色相落位 diff 三色空档、+n −m 统计 chips、行底色 tint；DiffTab 改用
  该栈（git diff tab 从此也有高亮）。
- 交互：两视角共用底部可拖拽预览面板（点击文件/提交/操作即预览，不再
  直接开 diff tab；面板可刷新、可键盘/拖拽调高、git 目标可一键展开为
  独立 diff tab）；视角选择与面板高度持久化在 tab.meta；git 行 XY 徽章
  按状态字母着色；选中行高亮。
- i18n：fileTrace* 十键重命名为 changes* 前缀，git 键值更新为「文件
  变动」，新增 7 键（视角/筛选/面板操作），全部 21 个词典同步。
- 结构：src/client/changes/（ChangesTab/GitLens/SessionLens/DiffPane/
  ops）+ src/client/diff/；GitView/DiffView/filetrace/ 删除；git 样式
  从 sidebar.module.css 迁至 changes.module.css。
- 测试：builtins 8→7（git tab 承接 badge pin）、diff-rows（unified
  转换/mod 配对/间隙折叠/统计）、changes-tab（壳交互 jsdom）、
  diff-files-collapse（原 DiffView 折叠行为钉住）、changes-ops（原
  filetrace 数据层）、git-view-worktree 迁移至 GitLens；全量 1122 通过。
- 文档：README ×2 功能矩阵合并、external-plugin-guide §4.4 与内置清单
  7 tab、mount e2e 菜单标题同步。

src/client/changes/ChangesTab.tsx

@@ -0,0 +1,127 @@
+/**
+ * The unified changes tab: one tab, two lenses on "what changed?" — Git
+ * (repository truth: staged/unstaged files, commit box, history) and the
+ * session round (agent truth: every file the model read, wrote, or edited).
+ * Both lenses preview their selections in a shared resizable bottom pane
+ * ({@link DiffPane}); git targets can expand into the dedicated diff tab.
+ * The active lens and the pane height persist in the tab's meta, so the tab
+ * reopens exactly where it was left.
+ */
+import { useState } from 'react'
+import type { TabComponentProps } from '../service.ts'
+import { t } from '../locales.ts'
+import type { SidebarDiffRef } from '../state.ts'
+import { GitLens } from './GitLens.tsx'
+import { SessionLens } from './SessionLens.tsx'
+import { DiffPane, diffTabOf, type ChangesPreview } from './DiffPane.tsx'
+import type { FileOp } from './ops.ts'
+import css from './changes.module.css'
+
+/** The default preview pane height (px) before the first drag. */
+const PANE_HEIGHT_DEFAULT = 300
+
+type Lens = 'git' | 'session'
+
+/** The persisted tab meta (JSON-serializable; rides the layout). */
+interface ChangesMeta {
+  lens?: Lens
+  previewH?: number
+}
+
+export function ChangesTab({ ctx, store, scope, tab, visible, onOpenFile, onOpenDiff }: TabComponentProps) {
+  const meta = (tab.meta ?? {}) as ChangesMeta
+  const [lens, setLens] = useState<Lens>(meta.lens === 'session' ? 'session' : 'git')
+  const [preview, setPreview] = useState<ChangesPreview | null>(null)
+  const [paneHeight, setPaneHeight] = useState<number>(
+    typeof meta.previewH === 'number' && meta.previewH >= 140 ? meta.previewH : PANE_HEIGHT_DEFAULT,
+  )
+
+  /** Persist a meta patch onto the tab (lens choice, pane height). */
+  const patchMeta = (patch: ChangesMeta): void => {
+    ctx.get('betterSidebar')?.updateTab(tab.id, {
+      meta: { ...(tab.meta as ChangesMeta | undefined ?? {}), ...patch },
+    })
+  }
+
+  const chooseLens = (next: Lens): void => {
+    if (next === lens) return
+    setLens(next)
+    patchMeta({ lens: next })
+  }
+
+  /** Preview one git change (worktree file or commit) from the Git lens. */
+  const previewGit = (ref: SidebarDiffRef): void => {
+    setPreview({ kind: 'git', ref })
+  }
+
+  /** Preview one session op (with its best-effort prior content snapshot). */
+  const previewOp = (path: string, op: FileOp, prior: string | undefined): void => {
+    setPreview({ kind: 'op', path, op, prior })
+  }
+
+  const previewKey = (target: ChangesPreview): string => target.kind === 'git'
+    ? (target.ref.kind === 'worktree'
+        ? `git:w:${target.ref.path}:${target.ref.staged ? 's' : 'u'}`
+        : `git:c:${target.ref.hashFull}`)
+    : `op:${target.op.callId}`
+
+  return (
+    <div className={css.root}>
+      <div className={css.lensBar}>
+        <div className={css.lensSwitch} role="group" aria-label={t('changes')}>
+          <button
+            type="button"
+            className={css.lensButton}
+            data-active={lens === 'git' ? 'true' : undefined}
+            aria-pressed={lens === 'git'}
+            onClick={() => { chooseLens('git') }}
+          >
+            {t('changesGitLens')}
+          </button>
+          <button
+            type="button"
+            className={css.lensButton}
+            data-active={lens === 'session' ? 'true' : undefined}
+            aria-pressed={lens === 'session'}
+            onClick={() => { chooseLens('session') }}
+          >
+            {t('changesSessionLens')}
+          </button>
+        </div>
+      </div>
+      {lens === 'git'
+        ? (
+          <GitLens
+            scope={scope}
+            store={store}
+            visible={visible}
+            onOpenFile={onOpenFile ?? (() => { /* no-op */ })}
+            onPreview={previewGit}
+            selectedRef={preview !== null && preview.kind === 'git' ? preview.ref : null}
+          />
+        )
+        : (
+          <SessionLens
+            ctx={ctx}
+            scope={scope}
+            visible={visible}
+            onPreview={previewOp}
+            selectedCallId={preview !== null && preview.kind === 'op' ? preview.op.callId : null}
+          />
+        )}
+      {preview !== null && (
+        <DiffPane
+          key={previewKey(preview)}
+          target={preview}
+          scope={scope}
+          height={paneHeight}
+          onHeightCommit={(height) => { setPaneHeight(height); patchMeta({ previewH: height }) }}
+          onClose={() => { setPreview(null) }}
+          onExpand={() => {
+            if (preview.kind === 'git') onOpenDiff?.(diffTabOf(preview.ref))
+          }}
+        />
+      )}
+    </div>
+  )
+}