feat(releases): add source archive fetchers for GitHub, Gitea, GitLab

For packages installed from auto-generated source tarballs rather
than uploaded binary assets (shell scripts, vim plugins, etc.).
Each delegates to its respective forge fetcher — the distinction
is organizational, signaling which fields the consumer should use.

internal/releases/githubsrc/githubsrc.go

@@ -0,0 +1,27 @@
+// Package githubsrc fetches source archives from GitHub releases.
+//
+// Some packages (shell scripts, vim plugins) are installed from the
+// auto-generated source tarballs rather than uploaded binary assets.
+// This package fetches releases and exposes the tarball/zipball URLs.
+//
+// Use [github] for packages that use uploaded binary assets.
+package githubsrc
+
+import (
+	"context"
+	"iter"
+	"net/http"
+
+	"github.com/webinstall/webi-installers/internal/releases/githubish"
+)
+
+const baseURL = "https://api.github.com"
+
+// Fetch retrieves releases from GitHub for the given owner/repo.
+// Paginates automatically, yielding one batch per API page.
+//
+// Callers should use [githubish.Release.TarballURL] and
+// [githubish.Release.ZipballURL] rather than the Assets list.
+func Fetch(ctx context.Context, client *http.Client, owner, repo string, auth *githubish.Auth) iter.Seq2[[]githubish.Release, error] {
+	return githubish.Fetch(ctx, client, baseURL, owner, repo, auth)
+}