docs: design contract and driver conventions

Design for a single-service replacement of Radarr/Sonarr: domain model,
policy engine, sourcing via Prowlarr Torznab, import pipeline, reconcile
loop, crate layout, CI gate and build order.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -0,0 +1,28 @@
+# arr
+
+One service that replaces Radarr and Sonarr, and later Bazarr, for a single
+household.
+
+Five .NET services idle at 600-900 MB to run a loop with four steps in it:
+decide what you want, find it, fetch it, put it where Jellyfin can read it.
+Each has a configuration surface an order of magnitude larger than the subset
+anyone uses, and none of them share a model.
+
+This is that loop, once, in Rust.
+
+- **Prowlarr** stays — it owns tracker auth, Cloudflare bypass and the indexer
+  definitions, and replacing it buys nothing.
+- **Transmission** stays.
+- **Jellyseerr** stays, talking to a thin Radarr-compatible shim.
+
+## Status
+
+Design complete, implementation not started. See `DESIGN.md` for the contract
+and the issue tracker for the work.
+
+## Documents
+
+- `DESIGN.md` — the contract. Domain model, policy engine, sourcing, import,
+  crate layout, build order.
+- `CLAUDE.md` — repository conventions, label taxonomy, and how an autonomous
+  driver session should sequence the work.