feat: improve documentation quality with real data

- Extract file-level docstrings from Python files (module-level string expressions)
- Use __init__.py docstrings as module doc_summary
- Use file docstrings as file purpose in layout tables (instead of 'Source file')
- Populate module outbound_modules/inbound_modules from import edges (internal only)
- Make filename sanitization consistent (sanitize_for_link matches sanitize_filename)
- Clean up stale .md files from previous runs before generating
- Fill ARCHITECTURE.md template with real layout, modules index, and critical points
- Add file_docstring field to ParsedModule and file_purpose to FileDoc

test-project/docs/architecture/modules/src.md

@@ -0,0 +1,26 @@
+# Module: src
+
+Test project package.
+
+## Symbols
+
+
+## Dependencies
+
+### Imports
+
+### Outbound Modules
+
+### Inbound Modules
+
+## Integrations
+
+
+
+
+## Usage Examples
+
+```python
+import src
+```
+