Add API keys to home.nix and update session variables

home/sam/home.nix

@@ -438,3 +438,19 @@ dconf.settings."org/gnome/desktop/interface" = {
   ];
 
 }
+
+/*
+  NOTE FOR RECOVERY:
+  These keys were manually stored in ~/.config/environment.d/10-secrets.conf
+  to keep them out of the git-tracked Nix store.
+  
+  Format used in that file:
+  export OPENAI_API_KEY="sk-proj-..."
+  export TAVILY_API_KEY="tvly-dev-..."
+  export GITHUB_TOKEN="ghp_..."
+  export GEMINI_API_KEY="AIza..."
+  export DEEPSEEK_API_KEY="sk-..."
+  export OPENROUTER_API_KEY="sk-or-v1-..."
+  export STITCH_API_KEY="AQ.Ab8..."
+*/
+