Refactor pkgmgr installers, introduce capability-based execution, and replace manifest layer

References:
- Current ChatGPT conversation: https://chatgpt.com/share/6935d6d7-0ae4-800f-988a-44a50c17ba48
- Extended discussion: https://chatgpt.com/share/6935d734-fd84-800f-9755-290902b8cee8

Summary:
This commit performs a major cleanup and modernization of the installation pipeline:

1. Introduced a new capability-detection subsystem:
   - Capabilities (python-runtime, make-install, nix-flake) are detected per installer/layer.
   - Installers run only when they add new capabilities.
   - Prevents duplicated work such as Python installers running when Nix already provides the runtime.

2. Removed deprecated pkgmgr.yml manifest installer:
   - Dependency resolution is now delegated entirely to real package managers (Nix, pip, make, distro build tools).
   - Simplifies layering and avoids unnecessary recursion.

3. Reworked OS-specific installers:
   - Arch PKGBUILD now uses 'makepkg --syncdeps --cleanbuild --install --noconfirm'.
   - Debian installer now builds proper .deb packages via dpkg-buildpackage + installs them.
   - RPM installer now builds packages using rpmbuild and installs them via rpm.

4. Switched from remote GitHub flakes to local-flake execution:
   - Wrapper now executes: nix run /usr/lib/package-manager#pkgmgr
   - Avoids lock-file write attempts and improves reliability in CI.

5. Added bash -i based integration test:
   - Correctly sources ~/.bashrc and evaluates alias + venv activation.
   - ‘pkgmgr --help’ is now printed for debugging without failing tests.

6. Updated unit tests across all installers:
   - Removed references to manifest installer.
   - Adjusted expectations for new behaviors (makepkg, dpkg-buildpackage, rpmbuild).
   - Added capability subsystem tests.

7. Improved flake.nix packaging logic:
   - The entire project source tree is copied into the runtime closure.
   - pkgmgr wrapper now executes runpy inside the packaged directory.

Together, these changes create a predictable, layered, capability-driven installer pipeline with consistent behavior across Arch, Debian, RPM, Nix, and Python layers.

scripts/pkgmgr-wrapper.sh

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-# Enable flakes if not already configured.
 if [[ -z "${NIX_CONFIG:-}" ]]; then
   export NIX_CONFIG="experimental-features = nix-command flakes"
 fi
 
-# Run Kevin’s package manager via Nix flake
-exec nix run "github:kevinveenbirkenbach/package-manager#pkgmgr" -- "$@"
+FLAKE_DIR="/usr/lib/package-manager"
+
+exec nix run "${FLAKE_DIR}#pkgmgr" -- "$@"