Documentation Standardisation & Unit Test Stabilization

- Overhauled README.md to serve as a unified system index and WIP tracker.
- Standardized documentation filenames (ARCH__, GUIDE__, PLAN__) for better discoverability.
- Archived completed project plans and scopes.
- Fixed regressions in unit tests (errors, models, email) caused by V3 architectural updates.
- Ensured unit tests remain non-destructive and environment-independent via mocking.

tests/unit/test_unit_errors.py

@@ -27,14 +27,16 @@ def test_error_formatting():
     formatted = format_db_error(raw)
     print(f"Raw: {raw}")
     print(f"Formatted: {formatted}")
-    if formatted == "Duplicate entry 'abc' for key 'id_random'":
+    
+    if formatted.category == "database_duplicate" and formatted.code == 1062:
         print("✅ Error formatting works.")
     else:
         print("❌ Error formatting FAILED.")
 
 def test_null_error_handling():
     print("\n--- Testing Null Error Handling ---")
-    if format_db_error(None) == "":
+    formatted = format_db_error(None)
+    if formatted.category == "unknown":
         print("✅ Null error handled correctly.")
     else:
         print("❌ Null error check FAILED.")