feat: Add JSON output and fail-on-severity for CI

[yardexx]

Description

• Add --format=json to analyze to print ProjectReport.toJson().
• Add --fail-on-severity=<info|warning|critical> to return non-zero exit codes when issues at or above the threshold exist.

Requirements

• Implement flag parsing and validation.
• JSON output mirrors ProjectReport.toJson().
• Exit codes: success when no issues at threshold; failure otherwise.
• Tests for both flags.
• README/docs updated with examples and CI snippets.

Additional Context

• Enables CI enforcement and machine-readable output.