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>

e2e-docker/caching_vcs_test.go

@@ -0,0 +1,7 @@
+//go:build dockere2e
+
+package e2edocker
+
+func init() {
+	cachingCases = append(cachingCases, cachingCase{"vcs", "octo/repo/archive/refs/tags/v1.0.0.tar.gz", "octo/repo/archive/refs/tags/v1.0.0.tar.gz"})
+}