Remove SSH_PROXY_ARCHITECTURE.md planning document

Consolidate documentation by removing the design/planning document
and keeping only the current-state description in ARCHITECTURE.md.

Author: Nils Bars

docs/ARCHITECTURE.md

@@ -68,8 +68,6 @@ Rust-based SSH proxy routing student connections to their containers.
 
 **Stack:** Rust + russh + tokio
 
-See `ssh-reverse-proxy/docs/SSH_PROXY_ARCHITECTURE.md` for detailed implementation.
-
 ### 4. ref-utils (`ref-docker-base/ref-utils/`)
 
 Python library for exercise submission testing, installed in all containers.