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).

src/client/changes/ops.ts

@@ -201,6 +201,25 @@ export function knownContentBefore(ops: readonly FileOp[], path: string, before:
   return undefined
 }
 
+/**
+ * Strip the DSH read-tool response envelope down to the file's raw content,
+ * PRESERVING blank lines (unlike {@link parseReadLines}, which drops them):
+ * drops the <content> wrapper, the "(Showing lines ...)" note, and the
+ * per-line "<n>: " prefixes. The markdown reading mode needs the blank lines
+ * because markdown block structure depends on them.
+ */
+export function parseReadContent(raw: string): string {
+  const contentMatch = raw.match(/<content>([\s\S]*?)<\/content>/)
+  const body = contentMatch ? contentMatch[1]! : raw
+  return body
+    .split('\n')
+    .filter((line) => !/^\s*\(Showing lines .*\)\s*$/.test(line))
+    .map((line) => line.replace(/^\s*\d+:\s/, ''))
+    .join('\n')
+    .replace(/^[\n]+/, '')
+    .replace(/\n+$/, '')
+}
+
 /** One parsed read line: the file's own line number and its content text. */
 export interface ReadLine { readonly line: number; readonly text: string }