Add ISO build tooling and serial console support

- Add bin/yino-iso with download, build, and clean subcommands for
  creating a preseeded Debian installer ISO
- Add --serial flag to bin/yino-vm boot for headless serial console
- Add offline installation ISO spec to FSD (section 6)
- Update README TODO to reflect ISO pipeline progress

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/yino-fsd.md

@@ -50,7 +50,18 @@ This should match Omarchy's architecture (see [Omarchy Analysis](Omarchy.md) for
 - QEMU should be used for testing and demo virtual machines
 - Keep a cache of downloaded assets (e.g. Debian installation ISO) in this repository's `download` directory
 
-## 6. Upstream tracking
+## 6. Offline installation ISO
+
+This project should generate the tooling necessary to create a modified, offline installation ISO.
+It should do this by downloading and caching the Debian iso-dvd image, extract it and apply the necessary
+modifications to enable an unattended installation with the following characteristics (similiar to
+how archinstall doest it):
+
+- LUKS + btrfs
+- System for a single user, automatic login because full disk decryption already authenticates user
+- Graphical Wayland environment, no X11
+
+## 7. Upstream tracking
 
 - Track the upstream [basecamp/omarchy](https://github.com/basecamp/omarchy) repository for new releases and changes
 - Omarchy's `dev` branch is where active development happens; `master` is the stable branch