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

archdoc-core/src/model.rs

@@ -51,6 +51,7 @@ pub struct FileDoc {
     pub outbound_modules: Vec<String>,
     pub inbound_files: Vec<String>,
     pub symbols: Vec<String>,
+    pub file_purpose: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -142,6 +143,7 @@ pub struct ParsedModule {
     pub imports: Vec<Import>,
     pub symbols: Vec<Symbol>,
     pub calls: Vec<Call>,
+    pub file_docstring: Option<String>,
 }
 
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]