Restructure repo layout, wiring src/ and packaging for local and distro builds

- Add dev runner main.py that prefers local src/ over installed pkgmgr
- Move Arch/Debian/Fedora packaging files under packaging/* and update build scripts
- Adjust .gitignore/.dockerignore for new packaging paths and src/source/
- Improve config defaults discovery to support src/ layout and installed packages
- Update architecture diagram and add TODO overview for TAGS/MIRROR/SIGNING_KEY

https://chatgpt.com/share/693a76a0-e408-800f-9939-868524cbef4d

main.py

@@ -1,6 +1,14 @@
 #!/usr/bin/env python3
+import sys
+from pathlib import Path
+
+# Ensure local src/ overrides installed package
+ROOT = Path(__file__).resolve().parent
+SRC = ROOT / "src"
+if SRC.is_dir():
+    sys.path.insert(0, str(SRC))
 
 from pkgmgr.cli import main
 
 if __name__ == "__main__":
-    main()
+    main()