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/control

@@ -0,0 +1,18 @@
+Source: package-manager
+Section: utils
+Priority: optional
+Maintainer: Kevin Veen-Birkenbach <info@veen.world>
+Build-Depends: debhelper-compat (= 13)
+Standards-Version: 4.6.2
+Rules-Requires-Root: no
+Homepage: https://github.com/kevinveenbirkenbach/package-manager
+
+Package: package-manager
+Architecture: any
+Depends: nix, ${misc:Depends}
+Description: Wrapper that runs Kevin's package-manager via Nix flake
+ This package provides the `pkgmgr` command, which runs Kevin's package
+ manager via a local Nix flake
+   nix run /usr/lib/package-manager#pkgmgr -- ...
+ Nix is the only runtime dependency and must be initialized on
+ the system to work correctly.