feat: add RubricConfig.load_bundle() for rubric bundle manifests

[luca-belli]

Summary

• Adds RubricConfig.load_bundle(), a classmethod that loads a rubric from a rubric bundle manifest JSON file (validates required keys, resolves relative paths against the manifest's folder, delegates to the existing RubricConfig.load())
• Adds data/rubric_manifest.json, the production default manifest pointing at the existing data/rubric.tsv bundle
• Adds tests/fixtures/rubric_manifest_simple.json, a test fixture manifest for the existing rubric_simple.tsv fixture

This is Phase 0 step 1 of 2 (helper only). A follow-up PR wires judge.py's currently-disconnected --rubrics flag to this helper.

Test plan

• uv run pytest -m "not live" passes (963 passed)
• New unit tests cover: successful load via manifest, FileNotFoundError on missing manifest, ValueError on missing required manifest key
• uv run ruff check / uv run ruff format --check clean