Add ElecTrace agent skill

[Austin243]

Summary

• add a compact public guide for coding agents
• ship an installable, plugin-ready electrace Codex skill
• keep detailed workflow boundaries in one progressive reference
• validate trigger metadata and include skill Markdown in documentation linting

Impact

Agents can install ElecTrace from the public repository, install the companion skill, route existing-output analysis versus calculation staging correctly, and preserve explicit execution/submission authorization.

Validation

• ruff check .
• ruff format --check .
• mypy src
• pytest -q — 347 passed
• mkdocs build --strict
• pre-commit run --all-files --show-diff-on-failure
• quick_validate.py skills/electrace
• three fresh-agent forward tests: existing results, review-only Slurm staging, and Chargemol/oxidation-state boundary