Add vcs remote type (#162)

The generic `github` remote only allowlists URL patterns: every repo needs hand-written mutable patterns for branch archives, and a moved branch re-downloads the whole archive under a ref-named key.

- add `vcs` package type for GitHub or Gitea (picked from `base_url`) tag, commit and release-download paths, cached immutable
- redirect `archive/refs/heads/<branch>` to the commit archive, resolving the branch via the branch API with one shared, per-remote bounded lookup cache reused for `mutable_ttl`
- cache missing branches for at least 60s and map API rate limits to `503` with `Retry-After`
- reject any other path shape, escaping segments and denied redirect targets
- add `docs/vcs/remote.md`, a UI type def and a vcs e2e caching case

Reviewed-on: #162
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

pkg/models/package_type_vcs.go

@@ -0,0 +1,5 @@
+package models
+
+const PackageVCS PackageType = "vcs"
+
+func init() { registerPackageType(PackageVCS) }