fix(cli): avoid panics when rendering agent JSON

[cedric013]

Problem

The agent CLI currently uses serde_json::to_string_pretty(...).expect(...) for JSON output paths. These code paths can panic instead of returning a normal CLI error if serialization fails.

Scope

• Replace the agent command JSON rendering expects with a small helper.
• Preserve existing CLI output shape on success.
• Report a clean agent error on serialization failure.

Validation

• rustfmt --check src/cli/agent_cmd.rs