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

packaging/debian/postinst

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -e
+
+case "$1" in
+    configure)
+        if [ -x /usr/lib/package-manager/init-nix.sh ]; then
+            /usr/lib/package-manager/init-nix.sh || true
+        else
+            echo ">>> Warning: /usr/lib/package-manager/init-nix.sh not found or not executable."
+        fi
+        ;;
+esac
+
+exit 0