Add TNEF handling and email loading improvements

- Implement TNEF extraction and recursive parsing in new `tnef_reader.go` and associated tests.
- Create tests for TNEF extraction scenarios in `tnef_diag_test.go`, `tnef_diag7_test.go`, and `tnef_diag8_test.go`.

.claude/settings.local.json

@@ -0,0 +1,13 @@
+{
+  "permissions": {
+    "allow": [
+      "WebSearch",
+      "WebFetch(domain:github.com)",
+      "WebFetch(domain:www.gnu.org)",
+      "Bash(go run:*)",
+      "Bash(go build:*)",
+      "Bash(go doc:*)",
+      "Bash(go test:*)"
+    ]
+  }
+}