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>

docs/common/caching.md

@@ -98,4 +98,4 @@ Each type's page lists its auto-detected mutable and immutable paths:
 [docker](../docker/remote.md), [generic](../generic/remote.md), [gems](../gems/remote.md), [goproxy](../goproxy/remote.md),
 [helm](../helm/remote.md), [npm](../npm/remote.md), [nuget](../nuget/remote.md),
 [puppet](../puppet/remote.md), [pypi](../pypi/remote.md), [rpm](../rpm/remote.md),
-[rpm github](../rpm/remote-github.md), [terraform](../terraform/remote.md).
+[rpm github](../rpm/remote-github.md), [terraform](../terraform/remote.md), [vcs](../vcs/remote.md).