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/files/src____init__.py.md

@@ -0,0 +1,28 @@
+# File: ./src/__init__.py
+
+- **Module:** src
+- **Defined symbols:** 0
+- **Imports:** 0
+
+<!-- MANUAL:BEGIN -->
+## File intent (manual)
+<FILL_MANUALLY>
+<!-- MANUAL:END -->
+
+---
+
+## Imports & file-level dependencies
+<!-- ARCHDOC:BEGIN section=file_imports -->
+> Generated. Do not edit inside this block.
+<!-- ARCHDOC:END section=file_imports -->
+
+---
+
+## Symbols index
+<!-- ARCHDOC:BEGIN section=symbols_index -->
+> Generated. Do not edit inside this block.
+<!-- ARCHDOC:END section=symbols_index -->
+
+---
+
+## Symbol details