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

pyproject.toml

@@ -39,13 +39,14 @@ pkgmgr = "pkgmgr.cli:main"
 # -----------------------------
 # setuptools configuration
 # -----------------------------
-# We use find_packages(), not a fixed list,
-# and explicitly include pkgmgr* and config*
+# Source layout: all packages live under "src/"
+[tool.setuptools]
+package-dir = { "" = "src", "config" = "config" }
 
 [tool.setuptools.packages.find]
-where = ["."]
+where = ["src", "."]
 include = ["pkgmgr*", "config*"]
 
-# Ensure defaults.yaml is shipped inside wheels & nix builds
 [tool.setuptools.package-data]
 "config" = ["defaults.yaml"]
+