feat(changes): markdown reading mode for .md op previews

The changes tab preview pane gains a Reading/Raw toggle for .md/.markdown/.mdx session ops (read, write, and edit alike). Reading mode renders the resulting document through the shared MarkdownText pass (the same stack as the editor preview): GFM tables with alignment, task lists, strikethrough, footnotes, math, and code fences, with local image destinations rewritten through the session-scoped /sidebar/file media route (rewriteLocalImageUrls). Content sources: read -> parseReadContent (envelope stripped, blank lines preserved — markdown block structure depends on them, which parseReadLines drops); write -> the written content; edit -> the reconstructed resulting document from the in-window prior content when known, else the new snippet. Ported from dsh-file-trace v0.2.0 (PR #471 lineage), re-based onto the unified changes tab (#475) and the repo shared-renderer conventions (markdownTextProps labels contract, skin tokens, 21-dictionary locale parity).

tests/changes-ops.spec.ts

@@ -4,7 +4,7 @@
  * (their full behavior suites live in the standalone dsh-file-trace plugin).
  */
 import { describe, expect, it } from 'vitest'
-import { extractFileOps, groupByFile, knownContentBefore, parseReadLines } from '../src/client/changes/ops.ts'
+import { extractFileOps, groupByFile, knownContentBefore, parseReadContent, parseReadLines } from '../src/client/changes/ops.ts'
 import { diffLines, buildDiffSegments, coalesceInline, diffInline } from '../src/client/diff/rows.ts'
 import { langOfPath, scanLine } from '../src/client/diff/highlight.ts'
 import type { SidebarSessionEvent } from '../src/context-types.ts'
@@ -29,6 +29,27 @@ function result(seq: number, callId: string, text: string, isError = false, time
   })
 }
 
+describe('parseReadContent', () => {
+  it('strips the envelope and line-number prefixes but keeps blank lines', () => {
+    const raw = [
+      '<path>notes.md</path>',
+      '<type>file</type>',
+      '(Showing lines 1 to 4)',
+      '<content>',
+      '1: # Title',
+      '2: ',
+      '3: para one',
+      '4: ',
+      '</content>',
+    ].join('\n')
+    expect(parseReadContent(raw)).toBe('# Title\n\npara one')
+  })
+
+  it('returns the raw text unchanged when no <content> section is present', () => {
+    expect(parseReadContent('plain text')).toBe('plain text')
+  })
+})
+
 describe('extractFileOps', () => {
   it('seeds running ops from tool/call and settles them from tool/result', () => {
     const ops = extractFileOps([