feat(addons): GitHub settings page, registration and docs

Add the GitHub connection page (fine-grained token, repository list,
read-only note, Test that checks each listed repository), let the shared
connector page hide messaging settings for a kind that carries no
messages, register the add-on in the package matrix and the catalog, and
document the credential slot and the GitHub add-on.

Assisted-by: Claude:claude-sonnet-5-5 [go node]

addons/github/ui/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "localai-github-ui",
+  "private": true,
+  "version": "0.1.0",
+  "description": "Native UI of the GitHub add-on: a standalone ES module that the host loads after its own build.",
+  "type": "module",
+  "scripts": { "build": "node build.mjs", "test": "node test/run.mjs" }
+}