docs: add KEEPER.md and CONTRIBUTING-CONTRACT.md

[my908-hue]

Closes #409
Closes #410

Changes

• docs/KEEPER.md — Keeper bot operations guide covering: what a keeper is and why it's needed, reference keeper script, configuration options, recommended cadence, monitoring/alerting (Prometheus rules + health endpoint), handling failed charges, deployment patterns (cron → systemd → HA leader election), and operational checklist.
• docs/CONTRIBUTING-CONTRACT.md — Contract contribution guide covering: module layout, how to add a new public function, testing conventions (naming, setup helper, event assertions, time advancement), benchmarks with baselines, event emission rules, error variant guidelines, and the full PR checklist.

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