Add reusable Andromeda OS engineering skill

[oratis]

Summary

• add a repository-versioned $andromeda-os-engineering skill covering research, architecture, implementation, hardware evidence, QEMU/GCP E2E, incident diagnosis, PR gating, and release verification
• capture reusable project memory in four progressively loaded references without storing secrets or session transcripts
• add read-only repository audit and exact-head merge-gate tools
• link the skill from the project README

Why

Andromeda work spans product research, Rust control-plane contracts, bootc installer constraints, hardware evidence boundaries, long-running E2E, and multi-PR release sequencing. The skill preserves the non-obvious invariants and makes the merge gates executable for future Codex sessions.

Validation

• official skill-creator quick_validate.py: pass
• relative Markdown link validation: pass
• Python syntax/import checks: pass
• negative gate test: required NEUTRAL conclusion is blocked
• real docs-only PR Fix doc/product consistency: SupportTier ladder, marker list, ADR template #11: correctly does not require Installable OS E2E
• real OS PR Harden OS/installer/CI: bootc observability, kernel-level taskd loopback, supply-chain pinning #13: correctly requires and verifies Installable OS E2E
• independent forward test: correctly identified Containerfile E2E requirements and led to stricter gate semantics
• git diff --check: pass