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>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "src/repos"]
+	path = src/repos
+	url = gitea@git.dannyhaslund.dk:danny8632/repos.git