docs: clarify repo topology and connected surfaces

Author: mdheller

README.md

@@ -3,3 +3,20 @@
 Immutable, local-first OS substrate (workstation + edge) with verifiable policy and user-space isolation.
 
 **No community automation dependency.**
+
+## Topology position
+
+- **Role:** immutable OS substrate for workstation and edge lanes.
+- **Connects to:**
+  - `SociOS-Linux/agentos-spine` — current Linux-side integration/workspace spine that assembles or routes adjacent layers around the substrate
+  - `SourceOS-Linux/sourceos-spec` — canonical typed contracts, JSON-LD contexts, and shared vocabulary for policy, assets, and events
+  - `SociOS-Linux/workstation-contracts` — workstation/CI contract and conformance lane
+  - `SociOS-Linux/socios` — opt-in automation commons, never a required dependency
+  - `SociOS-Linux/socioslinux-web` — Linux public web/docs surface that explains the substrate downstream
+  - Linux build and publish surfaces such as image/package lanes may realize this substrate, but this repo is the substrate rather than the builder
+- **Not this repo:**
+  - opt-in automation commons
+  - workspace controller
+  - public docs site
+  - canonical typed-contract registry
+- **Semantic direction:** this repo should eventually publish a substrate-focused repo descriptor that references the shared SourceOS/SociOS vocabulary from `sourceos-spec`.