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

.env.example

@@ -20,7 +20,7 @@ NVIDIA_API_KEY="your_nvidia_nim_key_here"
 
 # Cascade order (first available provider wins)
 # Default (if unset): openrouter,openai,anthropic,groq,gemini-api,deepseek,nvidia
-PROVIDER_CASCADE="deepseek,openrouter,openai,anthropic,groq,gemini,nvidia"
+PROVIDER_CASCADE=deepseek,openrouter,openai,anthropic,groq,gemini,nvidia
 
 # =============================================================================
 # LOCAL LLM (generic OpenAI-compatible endpoint)