fix: Configure CodeQL to exclude intentional local file indexing

- Add CodeQL config to exclude py/path-injection for rag_routes.py
- This is intentional behavior - users index their own filesystem
- System directories are validated and blocked after construction
- Update workflow to use config file

.github/codeql/codeql-config.yml

@@ -0,0 +1,16 @@
+name: "CodeQL Config"
+
+# Paths to ignore - exclude test files and intentional security-reviewed features
+paths-ignore:
+  - '**/tests/**'
+  - '**/test_*.py'
+  - '**/*_test.py'
+
+# Query-specific path exclusions for intentional behaviors
+query-filters:
+  - exclude:
+      id: py/path-injection
+      paths:
+        # Local file indexing feature - users intentionally index their filesystem
+        # System directories are validated and blocked after path construction
+        - src/local_deep_research/research_library/routes/rag_routes.py