feat: harden doctor json contract

[peg]

Summary

• harden rampart doctor --json behind an explicit rampart.doctor.v1 schema contract
• add generated/build metadata plus normalized summary/check result fields
• preserve compatibility with existing numeric warnings and issues, while exposing detailed warning_checks / issue_checks
• document the stable JSON contract in the CLI reference

Validation

• go test ./... -count=1
• go vet ./...
• make build
• git diff --check