[autofix] xiaohongshu/search: COMMAND_EXEC MutationObserver TypeError when document.body is null

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[autofix] xiaohongshu/search: COMMAND_EXEC MutationObserver TypeError when document.body is null

## Summary
Local autofix repair verified — retry passes the crash stage after this patch.

## Adapter
- Site: `xiaohongshu`
- Command: `xiaohongshu/search` (`clis/xiaohongshu/search.js`)
- OpenCLI version: `1.8.8`

## Failure
```
COMMAND_EXEC: Xiaohongshu search failed: TypeError: Failed to execute 'observe' on
'MutationObserver': parameter 1 is not of type 'Node'.
at :18:14
at new Promise ()
at :1:1
```
Reproduces consistently since ~2026-09-13 evenings (UTC+8), across all keywords,
even with default args. Trace `:18` maps to `WAIT_FOR_CONTENT_JS`
line `observer.observe(document.body, { childList: true, subtree: true })` —
`document.body` is null at evaluation time because the xiaohongshu search_result
page now performs an extra client-side redirect before committing a body, so the
evaluate lands in a document where only `documentElement` exists.

Note the same script already null-guards `document.body?.innerText` in `detect()`
two lines above — only the `observe()` target lacks the guard.

## Local fix
Guard both MutationObserver targets with a fallback (`document.body || documentElement`),
`search.js` lines 39 and 598:
```js
observer.observe(document.body || document.documentElement, { childList: true, subtree: true });
```
A subtree observer on `documentElement` fires when `` is inserted, so
content detection resumes normally. Also raised the content-wait ceiling from 5s
to 20s, which absorbs the extra redirect hop on slower sessions. Output shape
unchanged.

Related: #2470 (umbrella), #2445 (filter ambiguous_option — separate root cause,
same command).

_Issue filed by OpenCLI autofix after a verified local repair._

Start in clis/xiaohongshu/search.js, especially WAIT_FOR_CONTENT_JS around lines 39 and 598, and review the existing body guard in detect(). Run the xiaohongshu/search command with default and keyword arguments to reproduce the redirect timing, then verify the retry passes without a MutationObserver TypeError and preserves the existing output shape.

Written by the indexing model from the issue text.