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/.gitignore

@@ -0,0 +1,2 @@
+# Written by build.mjs; the package build makes it.
+/dist/