feat: add remediation confidence output schema

[kamalsrini]

Summary

• Add a normalized finding output schema in schemas/finding.schema.json.
• Require remediation recommendations to include confidence, blast radius, and behavior-change risk fields.
• Document the remediation output contract and update the skill template with the required shape.

Notes

• test_strategy remains optional here because UNI-15 owns mandatory test strategy requirements.

Verification

• ruby -rjson -e 'JSON.parse(File.read("schemas/finding.schema.json")); puts "OK: schemas/finding.schema.json parses"'
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• git diff --check

Linear: UNI-14