Add matrix_client_commet_scheme, like every other web client role has

The Commet role was the only playbook-installed web client without a
`_scheme` variable to go with its `_hostname` and `_path_prefix`, so
anything wishing to build Commet's public URL had to reach for
`matrix_playbook_ssl_enabled` itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

roles/custom/matrix-client-commet/defaults/main.yml

@@ -10,6 +10,9 @@ matrix_client_commet_enabled: true
 # The git branch, tag, or SHA to build from
 matrix_client_commet_version: "main"
 
+# The scheme used for serving Commet.
+matrix_client_commet_scheme: https
+
 # The hostname at which Commet is served (e.g. commet.example.com)
 matrix_client_commet_hostname: ""