docs: add repository health contract

[hummbl-dev]

Summary

• add docs/REPO_HEALTH.md for the active Arbiter package and quality-gate repo
• document package identity, validation commands, CI expectations, branch-protection expectations, and operational notes
• link the health contract from README.md

Verification

• git diff --check

Follow-up

• Opened Reconcile README install instructions with arbiter-score package metadata #71 for the README/package distribution name mismatch found during this pass.

Fleet tracker

• Part of Roll out docs/REPO_HEALTH.md contract across active hummbl-dev repos hummbl-dev#21

[hummbl-dev]

REVIEW VERDICT: APPROVE

No blocking findings. The PR adds a repo-health contract grounded in current package metadata, CI, reusable quality-gate workflow, and validation commands, and links it from the README. Local git diff --check passed. Hosted tests for Python 3.11/3.12 and self-grade are green. The separate package-name/install mismatch found during review is tracked in #71.

[hummbl-dev]

POST-MERGE SPOT CHECK: no blocking findings. Verified REPO_HEALTH claims against origin/main: pyproject.toml has package name arbiter-score, version 0.6.0, and arbiter CLI entry point; ci.yml runs Python 3.11/3.12 tests, separate tests/test_api.py, self-grade, and PR-only diff score; publish.yml, arbiter-quality-gate.yml, and weekly-leaderboard.yml exist and match the documented roles. Hosted #72 checks were green before merge.