feat(workflow): define runtime-neutral line-by-line review spec

[bntvllnt]

Summary

• require review mode to build a line-coverage ledger for every changed hunk
• add dedicated rules discovery and rule-by-rule coverage requirements
• require review output to include both human-readable summary and machine-readable artifact expectations
• define a runtime-neutral portable review spec plus optional executor patterns so the workflow skill stays OSS-safe and implementation-agnostic

What changed

• added workflow/references/reviews/core-portable-review-spec.md as the normative contract
• added workflow/references/reviews/executor-patterns.md for optional implementation patterns
• removed runtime/vendor references from the core portable spec and executor guidance
• updated review.md, review-output.md, and rules-discovery.md to align with the runtime-neutral contract
• updated workflow/SKILL.md, workflow/README.md, and CHANGELOG.md to reflect the new framing

Verification

• inspected the workflow diff and references end-to-end
• ran independent review passes on the documentation diff and fixed the blocking inconsistencies they found
• verified branch push and PR update via gh

Notes

• project-level rules remain the reproducible baseline for shared / CI review
• user-level rules are optional overlays when loaded and must be disclosed explicitly
• runtime-specific orchestration is not part of the core review contract
• named config-file conventions remain in rules discovery where they are necessary for interoperability