docs: add Codex handoff architecture docs

AGENTS.md

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+# AGENTS.md
+
+## Workflow
+
+- Inspect the repo before changing code.
+- Make minimal, targeted changes.
+- Preserve existing architecture.
+- Update docs when APIs, behavior, wiring, or hardware assumptions change.
+- Run available validation/tests before finishing.
+- Use meaningful commits.
+- Explain what changed, why, and any side effects.
+
+## Critical Constraints
+
+- Do not modify JBD/Xiaoxiang BLE behavior without explicit approval.
+- Do not change BLE timeout, reconnect, scan, or BMS-selection behavior without approval.
+- Cargo ESP32 remains the controller, API server, WebUI host, and source of truth.
+- Dashboard ESP32-S3 is a client only.
+- Do not move relay/BMS/control authority into the dashboard.
+- Do not hardcode install-specific names into firmware.
+- Preserve API compatibility when practical.
+- Prefer HTTP API integration over custom protocols.
+
+## Hardware Rules
+
+- ESP32 relay board outputs trigger external fused automotive relays/contactors.
+- Do not route fridge, Starlink, inverter, or other major load current through the ESP32 relay board.
+- Dashboard target is Waveshare ESP32-S3-Touch-LCD-5.
+- Dashboard connects to Cargo ESP32 AP over WiFi.
+- Dashboard is powered from ACC switched 12V into VIN.
+- Do not power dashboard from OBD-II pin 16 in the permanent install.
+- CAN termination must stay disabled when connected to vehicle CAN.
+
+## Documentation
+
+Primary docs:
+
+- docs/ARCHITECTURE.md
+- docs/HARDWARE.md
+- docs/ROADMAP.md
+- docs/API.md when API changes