feat(engineering): add Aident Skill

[lukeguo12210]

Summary

• What: add a new engineering/skills/aident-skill skill for operating Aident Loadout from agent environments.
• Why: gives agents a governed workflow for discovering capabilities, checking Vault connections, executing real-world actions, and verifying audit history.

Details

• Adds a repo-native SKILL.md with only name + description frontmatter.
• Uses aident-skill as the skill name/slug; Aident Loadout is the product surface described by the skill.
• Adds focused references for the CLI operating model, MCP client setup, and troubleshooting.
• Does not modify generated Codex/Gemini/plugin/index/count files; CONTRIBUTING says maintainers handle those after merge.
• Does not add scripts, so there are no --help script checks to run.

Checklist

• Targets dev branch
• SKILL.md frontmatter: name + description only
• Under 500 lines
• No scripts added
• Security audit: 0 critical/high findings

Validation

• git diff --check
• python3 engineering/skills/skill-security-auditor/scripts/skill_security_auditor.py engineering/skills/aident-skill --strict
• Cross-reference path checks for linked in-repo skills

Notes

• The legacy engineering/skills/skill-tester/scripts/skill_validator.py still expects older frontmatter fields plus a scripts directory, which conflicts with current CONTRIBUTING/CONVENTIONS guidance for new skills. I kept this contribution aligned with the current guide instead of adding placeholder metadata or fake scripts.