Buzz is advertised by cron but has no scheduler adapter

Summary

The generic cron CLI accepts buzz as a provider, but the scheduler has no Buzz delivery adapter.

As a result, users can successfully create a Buzz schedule that always fails during execution with Unsupported provider.

Reproduction

opensre cron add \
  --kind manual_loop \
  --cron "0 9 * * *" \
  --prompt "Check open incidents" \
  --provider buzz \
  --chat-id <channel-id>

The task is created successfully. When the scheduler fires, delivery fails because Provider.BUZZ is not present in the installed scheduled-delivery adapter bundle.

Expected behavior

Either:

1. implement scheduled Buzz delivery, or
2. remove buzz from generic cron provider choices and reject it clearly.

The CLI must not advertise providers that the scheduler cannot execute.

Affected files

• surfaces/cli/commands/cron.py
• bootstrap/adapters.py
• infrastructure/scheduling/scheduler/delivery_bundle.py
• infrastructure/scheduling/scheduler/types.py

Acceptance criteria

• Every provider shown by cron add --help has a working scheduler adapter.
• Unsupported providers are rejected during task creation.
• Add a provider-conformance test covering CLI choices and installed adapters.