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/rules

@@ -0,0 +1,55 @@
+#!/usr/bin/make -f
+
+%:
+	dh $@
+
+# ---------------------------------------------------------------------------
+# Disable automatic build + tests for Debian package
+#
+# The package is effectively a wrapper around the project source and the
+# Nix flake. We do not need dh_auto_build to invoke the top-level Makefile,
+# because that Makefile is meant for developer workflows (venv, setup, etc.)
+# and would fail in the minimal build environment.
+# ---------------------------------------------------------------------------
+override_dh_auto_build:
+	# No build step required for this package; skip calling the top-level Makefile.
+	:
+
+override_dh_auto_test:
+	# We do not run tests as part of the Debian package build.
+	:
+
+# ---------------------------------------------------------------------------
+# Install phase: copy wrapper + init script + full project source
+# ---------------------------------------------------------------------------
+override_dh_auto_install:
+	# Create target directories
+	install -d debian/package-manager/usr/bin
+	install -d debian/package-manager/usr/lib/package-manager
+
+	# Install wrapper
+	install -m0755 scripts/pkgmgr-wrapper.sh \
+		debian/package-manager/usr/bin/pkgmgr
+
+	# Install shared Nix init script
+	install -m0755 scripts/init-nix.sh \
+		debian/package-manager/usr/lib/package-manager/init-nix.sh
+
+	# Copy full project source into /usr/lib/package-manager,
+	# but do not include the debian/ directory itself.
+	find . -mindepth 1 -maxdepth 1 \
+	  ! -name debian \
+	  ! -name .git \
+	  -exec cp -a {} debian/package-manager/usr/lib/package-manager/ \;
+
+	# Remove packaging-only and development artefacts from the installed tree
+	rm -rf \
+	  debian/package-manager/usr/lib/package-manager/debian \
+	  debian/package-manager/usr/lib/package-manager/PKGBUILD \
+	  debian/package-manager/usr/lib/package-manager/Dockerfile \
+	  debian/package-manager/usr/lib/package-manager/.git \
+	  debian/package-manager/usr/lib/package-manager/.github \
+	  debian/package-manager/usr/lib/package-manager/tests \
+	  debian/package-manager/usr/lib/package-manager/.gitignore \
+	  debian/package-manager/usr/lib/package-manager/__pycache__ \
+	  debian/package-manager/usr/lib/package-manager/.gitkeep || true