Add portable loop engineering and evidence gate skills

[ideogram-fredtran]

Summary

• add a portable, script-first loop-engineering skill for Codex, Claude Code, and Cursor
• add Evidence Gate, a dogfooded companion skill that requires typed evidence for every completion criterion
• install canonical skills across Claude, Agents/Codex, and Cursor roots and cover the contracts with repository tests

Verification

• python3 /Users/fredtran/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/evidence-gate
• python3 /Users/fredtran/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/loop-engineering
• bash tests/run.sh all — 65 pass, 0 fail
• ruff format --check skills/evidence-gate skills/loop-engineering
• blind delegated Evidence Gate forward test: missing merge evidence exits 1 and emits no verification token

Dogfood

This delivery is running through loop-engineering state and an evidence-gate whose remaining criteria are the exact PR shape/check results and GitHub-confirmed merge into main. The loop cannot finish until both are recorded.