feat(remediation): require test strategy for fixes

[kamalsrini]

Summary

• Require test_strategy for every remediation in the normalized finding schema.
• Structure test_strategy as a summary plus recommended_tests, generated_tests, or both.
• Update remediation output docs and the skill template example/checklist.

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-15