add el-install binary and SDK bundle to release pipeline

- lang/tools/install/el-install.el: El program that fetches the latest
  release from the Gitea API, downloads el-sdk-latest.tar.gz, and
  extracts it into ~/.el (or a custom prefix passed as argv[1])
- lang/tools/install/manifest.el: build manifest for the el-install package
- .gitea/workflows/sdk-release.yaml: build elb, epm, and el-install
  binaries; bundle elc + elb + epm + runtime files into el-sdk-latest.tar.gz;
  attach both the tarball and el-install binary to the Gitea release
  alongside the existing per-file GCP uploads

lang/tools/install/manifest.el

@@ -0,0 +1,9 @@
+package "el-install" {
+    version "0.1.0"
+    description "El SDK installer"
+    edition "2026"
+}
+
+build {
+    entry "el-install.el"
+}