docs: hooks-guide auto-format example breaks on paths with spaces and exits 2 on unsupported files — contradicts the bundled update-config skill

## Summary

The "Auto-format code after edits" example in [hooks-guide](https://code.claude.com/docs/en/hooks-guide#auto-format-code-after-edits) has two defects. It appears twice on the page and is the first hook pattern most people copy.

```json
"command": "jq -r '.tool_input.file_path' | xargs npx prettier --write"
```

Both failures are silent or noisy in ways that surface long after the user has stopped watching the hook.

## Defect 1 — `xargs` word-splits paths containing spaces

`xargs` splits on whitespace, so one file becomes several nonexistent paths.

```console
$ echo '{"tool_input":{"file_path":"/tmp/carpeta con espacios/a.ts"}}' \
| jq -r '.tool_input.file_path' | xargs printf '[%s]\n'
[/tmp/carpeta]
[con]
[espacios/a.ts]

$ echo '{"tool_input":{"file_path":"/tmp/carpeta con espacios/a.ts"}}' \
| jq -r '.tool_input.file_path' | xargs ls
ls: /tmp/carpeta: No such file or directory
ls: con: No such file or directory
ls: espacios/a.ts: No such file or directory
```

Control: with a path that has no spaces, `xargs` works correctly. The bug is therefore intermittent by nature — the hook appears to work for months until someone edits a file under `My Project/`.

The page contains **no** warning about spaces or word-splitting. Searched the full 71 KB of the rendered page: zero occurrences of `read -r`, and nothing about quoting near the example. The only related note is 900 lines away under troubleshooting ("command not found"), suggesting `"args": []` for exec form.

## Defect 2 — no `--ignore-unknown`, so unsupported files exit 2

`exit 2` is the _blocking error_ code for hooks: stderr is fed back to Claude.

```console
$ prettier --write s.py ; echo "exit=$?"
[error] No parser could be inferred for file ".../s.py".
exit=2

$ prettier --write --ignore-unknown s.py ; echo "exit=$?"
exit=0
```

The documented command has neither `--ignore-unknown` nor `|| true`. In any polyglot repository, every edit to a `.py`, `.sh`, `.toml`, or any file Prettier has no parser for makes the hook return a blocking error on an operation that succeeded.

## Defect 3 (minor) — `npx` can hit the network

`npx prettier` with no local copy resolves by downloading from the registry. On a machine whose global Prettier lives inside an nvm release, a Node version bump removes it from `PATH` and this hook silently starts fetching Prettier on every edit.

## This contradicts a bundled first-party source

The `update-config` skill that ships with Claude Code prescribes the opposite, explicitly:

> use `jq -r` into a quoted variable or `{ read -r f; ... "$f"; }`, **NOT** unquoted `| xargs` (splits on spaces)

Two official sources of the same product disagree, and the more visible one is the incorrect one.

## Suggested fix

```json
"command": "jq -r '.tool_response.filePath // .tool_input.file_path' | { read -r f; npx prettier --write --ignore-unknown \"$f\"; } || true"
```

- `read -r` into a quoted variable — no word-splitting
- `--ignore-unknown` — unsupported files are skipped silently instead of exiting 2 (also honours `.prettierignore`)
- `|| true` — a formatter should not turn a successful edit into a hook error
- `.tool_response.filePath` first, matching the fallback the `update-config` skill uses

A one-line note that paths may contain spaces would also help readers who adapt the pattern to other formatters.

## Environment

- Claude Code 2.1.237
- macOS (arm64), APFS
- Prettier 3.9.6
- `jq` via Homebrew

## Also worth checking

The page states "When the hook succeeds, Claude Code shows nothing in the conversation." In practice a `PostToolUse` hook that rewrites the edited file surfaces a `PostToolUse hook modified after your edit (likely a formatter)` notice. I have not isolated the exact conditions, so this is an observation rather than a confirmed claim — but the sentence may be worth revisiting, since that notice is genuinely useful and users are being told it does not exist.

Start at the “Auto-format code after edits” example in hooks-guide, which appears twice, and compare it with the bundled update-config skill’s quoting guidance. Reproduce the reported space-containing path and unsupported-file cases, then update both copies so paths are preserved and unsupported files do not create blocking hook errors; verify the surrounding success wording against confirmed behavior.

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