Add Codex skill package

[Hanjo92]

Track adding a repository-native Codex skill package so this repo can be installed as a global Codex skill.\n\nScope:\n- Add skills/codex-goal-parser/SKILL.md\n- Bundle required CLI/reference resources under that skill folder\n- Document the install command\n- Verify the skill can run from its bundled script

[Hanjo92]

Added the repository-native Codex skill package.\n\nBranch and PR:\n- Branch: codex/add-skill-package\n- Commit: a52a08b add Codex skill package\n- Draft PR: https://github.com/Hanjo92/codex-goal-parser/pull/10\n\nFiles added:\n- skills/codex-goal-parser/SKILL.md\n- skills/codex-goal-parser/scripts/codex-goal-parser.js\n- skills/codex-goal-parser/references/output-contract.md\n- skills/codex-goal-parser/references/codex-goal-decompose.md\n\nAlso updated README with the skill-installer command.\n\nVerification:\n- npm test passed\n- bundled skill CLI produced valid JSON contract output\n- skill-installer installed the skill from the remote branch into a temporary destination\n- installed temporary skill CLI produced valid JSON contract output\n- GitHub Actions passed on Node 18 and Node 20