Add normalized JSON output contract

[kamalsrini]

Summary

• Add a SARIF-independent normalized JSON envelope with schema_version, run metadata, skill metadata, and findings.
• Document the normalized output contract in docs/normalized-json-output.md and link it from README/CONTRIBUTING/remediation docs.
• Require all 45 skill entrypoints to make findings available as JSON validating against schemas/finding.schema.json when machine-readable output is requested.

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-18