docs(packages): add README to core, graders, and reporter

[frederikprijck]

What

Add a README.md to the three published packages that lack one: @a0/evals-core, @a0/evals-graders, and @a0/evals-reporter. @a0/evals already has a comprehensive README and is unchanged.

Why

npm auto-includes a package's own README in its tarball (it does not pull one from the monorepo root). Today only @a0/evals ships documentation; the other three would publish with none. Each new README describes the package's purpose, its main exports, and a short usage example, with links back to the monorepo for the full guide.

Example snippets were checked against the actual exported signatures (contains/judge in graders, renderHtml/loadScores in reporter).

Split out from the packaging cleanup so README and license/notice changes review independently.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 88fa3ce4-ed64-434a-afcf-a8c22a6b7a3f

📥 Commits

Reviewing files that changed from the base of the PR and between 3e91dfe and 81923f0.

📒 Files selected for processing (3)

• packages/evals-core/README.md
• packages/evals-graders/README.md
• packages/evals-reporter/README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/package-readmes

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