feat: add Go release cache daemon (webicached)

Rewrites the Node.js release classification pipeline in Go. webicached
fetches upstream releases (GitHub, Gitea, GitLab, HashiCorp, custom
sources), classifies assets by OS/arch/variant, and writes legacy-format
JSON caches compatible with the existing webinstall.dev API.

Git-clone packages emit git_tag and git_commit_hash from real repo
clones — no fabricated refs.

internal/releases/github/github.go

@@ -0,0 +1,22 @@
+// Package github fetches releases from the GitHub API.
+//
+// This is a thin wrapper around [githubish] that sets the base URL to
+// https://api.github.com. Use [githubish] directly for Gitea, Forgejo,
+// or other GitHub-compatible forges.
+package github
+
+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.
+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)
+}