feat(agents): rewrite find-docs onto the nixpkgs ctx7 CLI

Assisted-by: DeepSeek Harness (claude-opus-5-5, deepseek-flash, qwen3.8-flash-next iq3)

hosts/idols-ai/preservation.nix

@@ -182,7 +182,16 @@ in
         ".local/share/opencode"
         ".local/state/opencode"
 
-        ".context7" # up-to-date docs and code examples for for LLMs & agents
+        # ctx7 docs lookup login
+        {
+          directory = ".config/context7";
+          mode = "0700";
+        }
+        # Legacy ctx7 login, still read: ctx7 cannot rename it across the two bind mounts.
+        {
+          directory = ".context7";
+          mode = "0700";
+        }
 
         # nvim
         ".local/share/nvim"