Add Claude Code hook to auto-run rustfmt after edits

PostToolUse hook runs rustfmt on any .rs file after Edit/Write,
ensuring Claude-generated Rust code is always properly formatted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/settings.json

@@ -0,0 +1,15 @@
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "FILE=$(jq -r '.tool_input.file_path'); if [[ \"$FILE\" == *.rs ]]; then rustfmt \"$FILE\" 2>/dev/null; fi"
+          }
+        ]
+      }
+    ]
+  }
+}