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]

.github/workflows/addons-package.yml

@@ -107,7 +107,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        addon: [tables, voice, face, fine-tuning, quantization, studio, talk, telegram, slack, office, sandbox, web, memory]
+        addon: [tables, voice, face, fine-tuning, quantization, studio, talk, telegram, slack, github, office, sandbox, web, memory]
     steps:
       - uses: actions/checkout@v7
         with:
@@ -226,7 +226,7 @@ jobs:
           # event it fails the plan. A missing or malformed platform file, or
           # any other error, fails the plan on every event.
           include='[]'
-          for id in tables voice face fine-tuning quantization studio talk telegram slack office sandbox web memory; do
+          for id in tables voice face fine-tuning quantization studio talk telegram slack github office sandbox web memory; do
             cfg="addons/$id/package/platforms.yaml"
             err="matrix-$id.err"
             if rows=$(go run ./scripts/addons/cmd/addonpkg matrix --config "$cfg" 2>"$err"); then