Docs — Explain validation behavior, toggles, and endpoints usage

[thatkookooguy]

Goal
Document Phase 01 behavior and usage to reduce onboarding friction.

Acceptance criteria

• Validation docs: warn-by-default behavior; fail-hard toggle (env/CLI); example messages
• Endpoint docs: GET /config (sanitized response), GET /metrics.json (fields, examples)
• README gets concise pointers to full docs

Notes

• Product-focused: implementation tech is intentionally unspecified
• Plan: /.cursor/plans/m-70793e-9f5a8969.plan.md (Phase 01)