Add JSON output format for gitintel stats

[arunroyjhs]

Summary

gitintel stats currently outputs human-readable tables. Adding a --format json flag would enable piping output to jq, dashboards, CI/CD pipelines, and other tooling.

What needs to happen

1. Add a --format flag to the stats subcommand (default: table, options: json, csv)
2. Structure the JSON output to include all current stats fields
3. Ensure backward compatibility — default output unchanged
4. Add integration tests for JSON output parsing

Example output

{
  "summary": {
    "total_commits": 234,
    "ai_commits": 47,
    "ai_percentage": 20.1,
    "agents": {
      "claude-code": 31,
      "cursor": 12,
      "copilot": 4
    }
  },
  "period": "30d",
  "scanned_at": "2026-03-29T12:00:00Z"
}

Good first issue because

• Isolated to the output formatting layer
• Rust's serde_json makes this straightforward
• Doesn't touch core scanning logic