Populate design, architecture and repo layout docs

project-yukart becomes the code-free hub: overview, roadmap and cross-cutting
design only. Each app or app cluster gets its own repo, indexed from here.

- ARCHITECTURE.md: separate services over a message bus, with displays as bus
  subscribers so the directly-driven screen, Android client and ESP32 panel
  share one contract. Records CAN scope (listen-only + OBD-II as sole writer)
  and a decisions table for choices still in flux.
- REPOS.md: repo splitting rule, planned repos, naming, and Gitea template-repo
  policy (scaffolding only — no upstream link after instantiation).
- ROADMAP.md: phases 0-5, capture and decode before displays.

Bus choice (MQTT) is marked proposed pending a check against real CAN rates;
implementation language is still open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -1,10 +1,37 @@
 # Project Yukart
 
-Mostly passive probing and info extraction and GUI on a Z27AG Colt Ralliart.  
-Overview available at [OVERVIEW.md](OVERVIEW.md).  
+Making a 2008 Mitsubishi Colt Ralliart (Z27AG) a bit smarter with modern technologies.
+What the system is meant to do is described in [OVERVIEW.md](OVERVIEW.md).
 
-## Other repositories
+This repository is the **overview and roadmap only — it holds no code.**
+Every application or application cluster lives in its own repository and is
+indexed here.
 
-- [Environment setup](gitea:yukart/orin-nano-env-setup)
+## Documents
 
-## Roadmap
+- [Overview](OVERVIEW.md) — what the system is meant to do
+- [Architecture](docs/ARCHITECTURE.md) — system shape, services, message bus, display contract
+- [Repositories](docs/REPOS.md) — repo layout, naming, template-repo policy
+- [Roadmap](docs/ROADMAP.md) — phases and current status
+
+## Repository index
+
+| Repository | Role | Status |
+|---|---|---|
+| [project-yukart](gitea:yukart/project-yukart) | This hub — overview, roadmap, cross-cutting design | Active |
+| [orin-nano-env-setup](gitea:yukart/orin-nano-env-setup) | Jetson host bring-up: drivers, kernel modules, OS-level setup | Active |
+
+Planned repositories and the reasoning behind the split are described in
+[docs/REPOS.md](docs/REPOS.md).
+
+## Hardware at a glance
+
+| Part | Role |
+|---|---|
+| NVIDIA Jetson Orin Nano Super (`mikkeli-orin-nano-2`) | Main compute, JetPack r36.4 / Ubuntu 22.04 |
+| PEAK PCAN-USB | Car CAN bus access (`can1`, 500 kbps) |
+| Onboard MTTCan | Second CAN interface (`can0`) |
+| Waveshare SIM7600G-H-M.2 | LTE Cat 4 + GNSS |
+
+Details and rationale live in the environment repo's
+[bill of materials](gitea:yukart/orin-nano-env-setup).