Workflow owner attribution should not create inbox mentions

## Problem
Workflow-generated channel messages currently use the owner attribution tag as if it were an explicit mention. That can place routine channel automation in the workflow owner's Inbox even when the message never @mentions them.

## Expected behavior
- preserve owner attribution for audit/ownership
- create mention recipients only from explicit message mentions
- repair existing derived mention rows produced solely from workflow ownership

I have a focused relay/workflow/database fix with a migration and tests ready to propose.

Start by tracing the relay and workflow message handling, then inspect the database models or queries for owner attribution and derived mention rows. Review the proposed migration and existing tests mentioned in the issue. Done means ownership remains available for audit, only explicit mentions create recipients, and rows created solely from workflow ownership are repaired.

Written by the indexing model from the issue text.