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/layout.md

@@ -0,0 +1,18 @@
+# Repository layout
+
+<!-- MANUAL:BEGIN -->
+## Manual overrides
+- `src/app/` — <FILL_MANUALLY>
+<!-- MANUAL:END -->
+
+---
+
+## Detected structure
+<!-- ARCHDOC:BEGIN section=layout_detected -->
+> Generated. Do not edit inside this block.
+| Path | Purpose | Link |
+|------|---------|------|
+| ./src/__init__.py | Test project package. | [details](files/src____init__.py.md) |
+| ./src/utils.py | Utility functions for the test project. | [details](files/src__utils.py.md) |
+| ./src/core.py | Core module with database and HTTP integrations. | [details](files/src__core.py.md) |
+<!-- ARCHDOC:END section=layout_detected -->