Debug — GET /config returns sanitized effective config

[thatkookooguy]

Goal
Provide a debug endpoint to return the effective configuration without secrets.

Acceptance criteria

• GET /config returns sanitized effective config (no secrets/headers)
• Response shape documented; e2e test validates contract
• Default off if needed by config flag; additive behavior

Notes

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