Add end-to-end GitHub issue delivery skill

[azunaVT]

Summary

• add a repository-scoped Codex skill for taking GitHub issues from intake to release readiness
• require isolated git worktrees for implementation
• define human gates for direction, local testing and publishing, and final merge
• require an independent read-only senior subagent review before handoff
• add UI metadata for skill discovery and invocation

Validation

• python3 /home/vscode/.codex/skills/.system/skill-creator/scripts/quick_validate.py .agents/skills/deliver-github-issue
• forward-tested with a fresh read-only subagent against issue Add Current user/sessions plumbing #13; it inspected the issue and stopped at Gate 1 without creating a worktree or changing local or remote state

Notes

This is an instruction-only workflow skill. Application tests are not affected.