docs(readme): document zero-divergence intent for per-host pins (#1)

README.md

@@ -4,7 +4,21 @@ Shared nixpkgs pin for the server fleet — ensures binary cache alignment
 
 ## Overview
 
-Centralized nixpkgs version pin shared by all servers in the fleet. Ensures consistent package versions and binary cache hits across armer, howard, emmett, micron, bim, and mermaid.
+Centralized nixpkgs version pin shared by all servers in the fleet. Ensures
+consistent package versions and binary cache hits across armer, howard, emmett,
+micron, bim, and mermaid.
+
+## Design intent
+
+All per-host pins (`nixpkgs-armer`, `nixpkgs-emmett`, etc.) currently
+`follows = "nixpkgs"` — there is **zero divergence** across the fleet today.
+This is intentional: a single shared revision maximises binary-cache hit rate
+and keeps the fleet in a known-good, consistent state.
+
+The per-host inputs exist as **named extension points** so that any individual
+host can be broken away from the fleet default by changing its `follows` to a
+direct `url`. Until a host actually needs to diverge, the pins simply track
+the default.
 
 ## Usage