CLI polish: output modes (json, table), file output, and --target support

**ID:** cli.polish
**Priority:** P3
**Component:** main.rs

Improve CLI UX for direct Prometheus troubleshooting and parity with MCP tools.

### Acceptance Criteria
- --output {json,table}
- --out path to write payloads
- --target for multi-prometheus config
- Basic tests

**Estimate:** 3
