fix: cl-dotenv quote contamination breaks provider cascade parsing

cl-dotenv preserves surrounding quotes in .env values (unlike bash).
PROVIDER_CASCADE="deepseek,..." resulted in keywords like :"DEEPSEEK
instead of :DEEPSEEK, causing all cascade lookups to fail silently.

Fixes:
- .env.example: remove quotes from PROVIDER_CASCADE
- provider-cascade-initialize: add #" and #' to string-trim chars
- system-model-router: same fix for LOCAL_BACKENDS parsing

lisp/system-model-provider.lisp

@@ -81,7 +81,7 @@
   (let ((cascade-str (uiop:getenv "PROVIDER_CASCADE")))
     (if cascade-str
         (setf *provider-cascade*
-              (mapcar (lambda (s) (intern (string-upcase (string-trim '(#\Space) s)) :keyword))
+              (mapcar (lambda (s) (intern (string-upcase (string-trim '(#\Space #\" #\') s)) :keyword))
                       (uiop:split-string cascade-str :separator '(#\,))))
         (setf *provider-cascade* (mapcar #'car (remove-if (lambda (e)
                                                              (member (car e) '(:local)))