Expose declared roles in agent roster JSON

[myobie]

Summary

• Add nullable role to both agent roster JSON shapes.
• Preserve the exact free-form Agent Spec role value.
• Update the stable wire contract documentation.

JSON shape

The summary form adds role after description. The enriched form adds the same field and keeps lastActivity and inbox.

Tests

• cargo check --tests
• cargo test --workspace --lib -- --test-threads=1
• cargo test --test status_agents -- --test-threads=1
• Focused Clippy with current-main allowances

[myobie]

Closing this — it was opened before its owner had approved the idea, which is our mistake and not something anyone else should spend time on.

The branch agent/agents-json-role stays and the work is intact at a79432d. If we decide we want this, it comes back as a pull request with a proper description of why.

Apologies to anyone who read it. Nothing here needed review.