fix(autofix-pr): extractBetween 支持 latest tag 截断时回溯到更早完整对

如果远端 agent 重试时写了完整 <autofix-result> 后又开了一个被截断的
第二个 tag, 旧实现只看 lastIndexOf(open) 然后找不到 close 就返回 null,
导致前面那个完整结果被丢弃。改为从尾向首遍历所有 open tag, 返回第一个
能配对的 open/close 对。

附带:
- docs/features/remote-agent-completion-analysis.md: 9 处裸 fenced block
  补 language tag (text/http), 修复 markdownlint MD040 警告
- 同文件: 两处"三选项" → "三个选项" 符合中文量词习惯

src/commands/autofix-pr/__tests__/extractAutofixResult.test.ts

@@ -89,6 +89,16 @@ describe('extractAutofixResultFromLog', () => {
     expect(extractAutofixResultFromLog(log)).toBeNull()
   })
 
+  test('returns earlier complete tag when latest open tag is truncated within the same block', () => {
+    // Retry scenario: a full result was emitted, then a second result tag
+    // started but got cut off. We should surface the earlier complete pair
+    // rather than dropping the whole block.
+    const complete = sampleTag('earlier complete result')
+    const truncated = `<${AUTOFIX_RESULT_TAG}>\n<summary>truncated retry...`
+    const log = [assistantTextMessage(`${complete}\n${truncated}`)]
+    expect(extractAutofixResultFromLog(log)).toBe(complete)
+  })
+
   test('walks backwards so hook stdout from later in log wins over earlier assistant text', () => {
     const earlier = sampleTag('via assistant first')
     const later = sampleTag('via hook later')

src/commands/autofix-pr/extractAutofixResult.ts

@@ -71,14 +71,22 @@ export function extractAutofixResultFromLog(log: SDKMessage[]): string | null {
   return null
 }
 
+// Walks open tags from latest to earliest, returning the first complete
+// open/close pair. Guards against a truncated final tag shadowing an
+// earlier complete pair within the same text block (e.g., a retry wrote a
+// full result, then the model started a second tag that got cut off).
 function extractBetween(
   text: string,
   open: string,
   close: string,
 ): string | null {
-  const start = text.lastIndexOf(open)
-  if (start === -1) return null
-  const end = text.indexOf(close, start + open.length)
-  if (end === -1) return null
-  return text.slice(start, end + close.length)
+  let searchFrom = text.length
+  while (searchFrom >= 0) {
+    const start = text.lastIndexOf(open, searchFrom)
+    if (start === -1) return null
+    const end = text.indexOf(close, start + open.length)
+    if (end !== -1) return text.slice(start, end + close.length)
+    searchFrom = start - 1
+  }
+  return null
 }