feat: add tracker handoff format

[kamalsrini]

Summary

• Add a strict tracker handoff schema for DefectDojo, Jira, Linear, and equivalent issue trackers.
• Document required tracker-ready fields: title, severity, evidence, owner, SLA, and remediation.
• Add normalized finding mappings plus DefectDojo/Jira/Linear field mappings.
• Link the handoff contract from README, CONTRIBUTING, and the skill template.

Verification

• Parsed schemas/tracker-handoff.schema.json as JSON.
• Parsed embedded JSON examples in docs/tracker-handoff.md.
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• git diff --check

Linear: UNI-20