Disable Claude Code co-author attribution (#1575)

Adds a shared `.claude/settings.json` that disables Claude Code's
attribution:

```json
"attribution": { "commit": "", "pr": "" }
```

By default Claude Code adds a `Co-Authored-By: Claude` trailer to
commits. Claude can't sign the Google CLA, so this trailer breaks the
CLA check.

Also adds `.claude/settings.local.json` to `.gitignore` so personal
settings aren't committed.

Same change as Kaggle/kaggle-cli#1233.

Author: rosbo

.claude/settings.json

@@ -0,0 +1,6 @@
+{
+  "attribution": {
+    "commit": "",
+    "pr": ""
+  }
+}

.gitignore

@@ -2,4 +2,7 @@
 .idea/
 .vscode
 .mypy_cache
-.generated
+.generated
+
+# Claude Code
+.claude/settings.local.json