Add artifactapi_remote_github_deb resource

Add the metadata-only Debian/apt analog of artifactapi_remote_github_rpm,
exposing a GitHub repo's release .deb assets as a synthesized apt repository
with downloads redirected to a backing releases_remote.

- Register newRemoteResource("github_deb") and add NewRemoteGitHubDeb ctor
- Bump provider resource count 19 -> 20 and add the type name to test lists
- Add modelToAPI/metadata/constructor test coverage for github_deb
- Document github_deb in README and add an examples/ main.tf (goodtune/ghp)

examples/resources/artifactapi_remote_github_deb/main.tf

@@ -0,0 +1,44 @@
+terraform {
+  required_providers {
+    artifactapi = {
+      source  = "git.unkin.net/unkin/artifactapi"
+      version = "0.0.1"
+    }
+  }
+}
+
+provider "artifactapi" {
+  endpoint = "https://artifactapi.example.com"
+}
+
+# Backend generic remote that serves the actual .deb bytes from github.com.
+# Its patterns must allowlist the target repo's release-download assets.
+resource "artifactapi_remote_generic" "github" {
+  name     = "github"
+  base_url = "https://github.com"
+
+  patterns = [
+    "goodtune/ghp/releases/download/.*\\.deb$",
+  ]
+}
+
+# Metadata-only remote: scans goodtune/ghp releases for .deb assets and serves a
+# synthesized apt repo. Packages are never precached; downloads 302-redirect to
+# the "github" remote above.
+resource "artifactapi_remote_github_deb" "goodtune_ghp" {
+  name            = "goodtune-ghp"
+  base_url        = "https://api.github.com/repos/goodtune/ghp"
+  description     = "goodtune/ghp GitHub releases as an apt repo"
+  releases_remote = artifactapi_remote_generic.github.name
+
+  mutable_ttl = 3600
+
+  # Optional: restrict which release assets become packages.
+  patterns = [
+    ".*_amd64\\.deb$",
+    ".*_all\\.deb$",
+  ]
+
+  # Optional: token for private repos / higher GitHub API rate limits.
+  # password = var.github_token
+}