fix: properly organize shared test resources

- Move fixtures/ and unit/ to shared/ directory where they belong
- These test resources are used by both npm and native installations
- Update package.json test scripts to use new paths
- Fix relative import path in helpers.test.js
- All 46 npm-based tests now passing (7 unit + 5 integration + 34 npm)

Final test structure:
- native/     - Native installation tests only
- npm/        - npm package tests only
- shared/     - Shared utilities, fixtures, and unit tests
- integration/- Cross-installation integration tests
- edge-cases.sh/.ps1 - Master orchestrators

tests/shared/fixtures/test-settings.json

@@ -0,0 +1,6 @@
+{
+  "env": {
+    "ANTHROPIC_BASE_URL": "https://api.example.com",
+    "ANTHROPIC_AUTH_TOKEN": "test-token"
+  }
+}