[P2] Attack contribution guide

[Petsku01]

Problem

No clear path for community attack submissions.

Tasks

• Add CONTRIBUTING.md section for new attacks
• Define required metadata fields
• Add validation script for attack format

[Petsku01]

Triage 2026-09-14, verified against main (79aafd0): mostly done already.

• CONTRIBUTING.md section for new attacks — exists (attack documentation section with detection/defense guidance)
• Define required metadata fields — psg/catalog_validator.py enforces them: required id/prompt (with aliases), optional technique, source, tier, attack_type
• Validation script for attack format — validator exists and is wired; what's missing is a documented submission path: where to put new catalog files, how to run the validator on a PR, and an example JSON

Proposal: narrow this issue to "add catalog-submission walkthrough (file location, validator invocation, example)" — the rest is shipped.