Add git sources, check-updates, repos submodule, improve docs

dpack features:
- Git source support: packages can specify [source].git for cloning
  instead of tarball download. Supports branch, tag, and commit pinning.
  SHA256 can be set to "SKIP" for git sources.
- check-updates command: queries upstream APIs (GitHub releases/tags)
  to find available updates. Packages set [source].update_check URL.
- CheckUpdates CLI subcommand wired into main.rs

Package changes:
- FreeCAD updated to weekly-2026.03.19 development builds
- dwl: added update_check URL and git source documentation
- src/repos extracted to standalone git repo (danny8632/repos.git)
  and added as git submodule

Documentation:
- All 7 README.md files updated with detailed requirements sections
  including which Linux distros are supported, exact package names
  for Arch/Ubuntu/Fedora, and clear notes about which components
  require Linux vs can be built on macOS
- dpack README: added git source and check-updates documentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/install/README.md

@@ -18,13 +18,20 @@ The installer walks through 9 steps to get a working DarkForge system on disk:
 
 ## Requirements
 
-The installer runs from the DarkForge live ISO environment. It expects:
+**Environment:** The installer runs exclusively from the DarkForge live ISO. It is a set of bash scripts designed for the DarkForge live environment and is not intended to be run from another distro or from macOS/Windows.
 
-- UEFI firmware (no legacy BIOS support)
-- At least one NVMe or SATA disk
-- `sgdisk` (GPT partitioning)
-- `mkfs.ext4`, `mkfs.fat`, `mkswap`
+**Hardware:**
+- UEFI firmware (no legacy BIOS support — the target machine must boot in UEFI mode)
+- At least one NVMe or SATA disk with enough space (minimum 250GB recommended)
+- Wired ethernet connection (for DHCP during post-install package downloads)
+
+**Tools provided by the live ISO:**
+- `bash` 5.x, `sgdisk` (GPT partitioning from gdisk package)
+- `mkfs.ext4` (from e2fsprogs), `mkfs.fat` (from dosfstools), `mkswap` (from util-linux)
 - `efibootmgr` (UEFI boot entry creation)
+- `dpack` (package manager — for base system installation)
+
+All of these are included in the live ISO. You do not need to install them separately.
 
 ## Usage