fix(security): address CodeQL path-traversal and info-exposure findings

- Add _validate_plugin_name() guard on all {name} path param endpoints
  (rejects /, \, .. before reaching plugin logic)
- Strip after_install_path from install response (no internal paths to client)
- Update nix/tui.nix lockfile hash to match committed package-lock.json

nix/tui.nix

@@ -4,7 +4,7 @@ let
   src = ../ui-tui;
   npmDeps = pkgs.fetchNpmDeps {
     inherit src;
-    hash = "sha256-Chz+NW9NXqboXHOa6PKwf5bhAkkcFtKNhvKWwg2XSPc=";
+    hash = "sha256-a/HGI9OgVcTnZrMXA7xFMGnFoVxyHe95fulVz+WNYB0=";
   };
 
   npm = hermesNpmLib.mkNpmPassthru { folder = "ui-tui"; attr = "tui"; pname = "hermes-tui"; };