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/fedora/package-manager.install

@@ -0,0 +1,11 @@
+post_install() {
+  /usr/lib/package-manager/init-nix.sh || true
+}
+
+post_upgrade() {
+  /usr/lib/package-manager/init-nix.sh || true
+}
+
+post_remove() {
+  echo ">>> package-manager removed. Nix itself was not removed."
+}