Add model-agnostic AI engineering harness

[anth-volk]

Fixes #1748

Summary

• Add AGENTS.md as the model-neutral AI guidance entry point.
• Convert CLAUDE.md and add Copilot instructions as thin adapters into shared docs.
• Add canonical UK engineering skills under docs/engineering/skills/ for PRs, testing, model structure, dataset sources, and documentation review.
• Document the required Towncrier changelog fragment workflow and required lint/format before committing.

Verification

• make format
• git diff --check