refactor: remove low-value maintenance hardcodes

[ssfun]

Summary

• derive Bun from each upstream Management package.json instead of maintaining a duplicate version
• remove unused overlay manifest metadata and align documentation with actual SHA validation
• group Gemini CLI quotas by stable model families and conservatively aggregate remaining quota

Validation

• PYTHONPYCACHEPREFIX=/private/tmp/cliproxyapi-pro-pycache bash scripts/validation/repo.sh
• Management v1.18.5: idempotence, 111 tests, lint, type-check, production build
• Core v7.2.95: drift preflight, reapply rejection, focused package tests, server build