docs(architecture): explain gateway session mapping and per-session queueing

docs/README.md

@@ -6,6 +6,7 @@ This documentation is written to be useful to both humans and AI agents. If you
 
 1. Architecture overview (agent-oriented)
    - `docs/architecture/AGENT_DIAGRAM.md`
+   - `docs/architecture/GATEWAY_SESSIONS_AND_QUEUE.md`
    - `docs/architecture/TYPESCRIPT_MAP.md`
    - `docs/architecture/SYMBOL_INDEX.md`
    - `docs/architecture/CONTRIBUTOR_MAP.md`
@@ -37,4 +38,3 @@ flowchart TD
   NA --> TP[ToolPolicy + ToolRegistry]
   TP --> TE[ToolExecutor\nhooks + enforcement + audit]
 ```
-