Scaffold repo: docs layout, guidelines and initial plan

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

docs/GUIDELINES.md

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+---
+type: default
+---
+# Gateway guidelines
+
+## 1. Read-only with respect to acquisition
+- The gateway subscribes. It never opens a CAN socket, TX-capable or otherwise.  
+- `obd-poller` is the single writer on the bus. Nothing here changes that.
+
+## 2. Aggregate, do not relay
+- A 500 kbps CAN bus produces far more traffic than a display can use or a USB
+  link should carry. Downsample and summarise here.  
+- If the gateway is only forwarding a topic unchanged, that client should have
+  subscribed to the broker directly instead.
+
+## 3. To be added