Add reusable engineering skills

[aryamantodkar]

Summary

• add reusable repository-local skills for code review, codebase design, domain modeling, design grilling, research, and TDD
• include each skill’s required references and Codex interface metadata
• adapt code-review to use the repository GitHub remote without the source repository bootstrapper
• update grill-with-docs to use current $skill-name references

Why

These skills provide shared engineering workflows that are generic across Premove repositories. Keeping them repository-local makes the guidance available to contributors working on premove-itn without introducing product code, automation workflows, or external runtime dependencies.

Validation

• all seven skills pass quick_validate.py
• all local Markdown instruction links resolve
• no source-repository paths or legacy bootstrap references remain
• git diff --check passes

Scope

This PR changes only .agents/skills/. It does not modify premove-itn product code, documentation, dependencies, or GitHub workflows.