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/iso/README.md

@@ -8,12 +8,25 @@ The ISO builder compresses the base system into a squashfs image, creates a UEFI
 
 ## Requirements
 
-- `mksquashfs` (squashfs-tools) — filesystem compression
-- `xorriso` — ISO9660 image creation
-- `mkfs.fat` (dosfstools) — EFI partition image
-- `mcopy` (mtools) — copy files into FAT images
-- A completed base system build (Phase 3)
-- A compiled kernel at `kernel/vmlinuz`
+**Environment:** Linux only. The ISO builder is a bash script that uses Linux-specific tools. It cannot run on macOS or Windows.
+
+**Host packages:**
+
+```bash
+# Arch Linux
+sudo pacman -S squashfs-tools xorriso dosfstools mtools
+
+# Ubuntu / Debian
+sudo apt install squashfs-tools xorriso dosfstools mtools
+
+# Fedora
+sudo dnf install squashfs-tools xorriso dosfstools mtools
+```
+
+**Build prerequisites:**
+- A completed base system build (Phase 3) in `build/base-system/`
+- A compiled kernel at `kernel/vmlinuz` (Phase 4)
+- The DarkForge toolchain chroot (Phase 0) or an equivalent build environment
 
 ## Usage