ci: quote e2e.yml input description so Gitea's YAML parser accepts it

Unquoted `(default: main)` trips Gitea Actions' strict YAML parser
with "mapping values are not allowed in this context" because the
inline colon reads as a nested mapping. GitHub Actions' parser was
lenient about this; Gitea's isn't.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/e2e.yml

@@ -10,7 +10,7 @@ on:
   workflow_dispatch:
     inputs:
       ref:
-        description: Git ref to test (default: main)
+        description: "Git ref to test (default: main)"
         required: false
         default: main