docs: add PR description skill

[illegalcall]

Code changes: +0 -0
Tests: +0 -0
Others: +40 -0

Summary

• add a model-invoked pr-description skill under .agents/skills
• point the repository PR hygiene rules at the skill so agents use it for every PR creation or update
• allowlist the curated skill while continuing to ignore locally-added personal skills

Validation

• python3 ~/.codex/skills/.system/skill-creator/scripts/quick_validate.py .agents/skills/pr-description
• GOFLAGS=-p=2 npm run lint
• npm run frontend:typecheck
• Docker-based npx @redwoodjs/agent-ci run --all was unavailable because the local Docker daemon is not running; remote PR checks cover this gap.

[i-trytoohard]

@illegalcall friendly nudge: if there's an issue this addresses, please link it (Fixes #N). If none exists, no action needed.