feat(installer): add one-file installer build and custom install ISO

Replace old_installer.py with a curses TUI installer packaged as a
one-file PyInstaller binary (python/installer/build.py, wrapped by
python/installer/package.nix). The default .#installer is patched to run
on foreign Linux live media; the new .#installer-nixos variant keeps its
Nix store interpreter so it runs on NixOS.

Add systems/iso, a minimal NixOS install CD with kernel 6.18 and ZFS 2.4
matching the deployed systems and the installer on PATH; build it with
nix build .#iso. Shared logging and subprocess helpers move out of
common.py into python/logging_config.py and python/process.py.

README.md

@@ -1 +1,51 @@
 # dotfiles
+
+## Installer ISO
+
+Build a bootable NixOS ISO with the installer preinstalled:
+
+```sh
+nix build .#iso
+```
+
+Write `result/iso/nixos-zfs-installer.iso` to a USB stick (for example with `dd`) or boot it in a VM. The image is the minimal NixOS installation CD with ZFS enabled and `nixos-installer` on `PATH`, so once booted you can run:
+
+```sh
+sudo nixos-installer
+```
+
+The ISO bundles the `.#installer-nixos` package, a variant of the binary that keeps its Nix store linkage instead of being patched for foreign distributions.
+
+## Installer binary
+
+Build the self-contained installer executable with:
+
+```sh
+nix build .#installer
+```
+
+The flake package (defined in `python/installer/package.nix`) uses the Python builder in `python/installer/build.py`, which stages only the installer modules before running PyInstaller. You can also call it directly when `pyinstaller` and `patchelf` are on `PATH`:
+
+```sh
+python -m python.installer.build --output ./nixos-installer
+```
+
+Copy `result/bin/nixos-installer` to the installer USB stick and run it as root from the NixOS live environment:
+
+```sh
+sudo ./nixos-installer
+```
+
+Validate the live environment first with:
+
+```sh
+./nixos-installer --check
+```
+
+Set `ENCRYPT_KEY` to enable LUKS during install:
+
+```sh
+sudo env ENCRYPT_KEY='change-me' ./nixos-installer
+```
+
+The binary bundles the Python runtime and only the installer modules it imports. It still expects the NixOS installer environment to provide system install tools such as `parted`, `zfs`, `zpool`, `cryptsetup`, `nixos-generate-config`, and `nixos-install`.