feat(v0.2.0): unified OpenAI-compatible LLM backend

Replace Ollama-specific backend with unified org-skill-unified-llm-backend
that speaks OpenAI API. Works with:
- Local: Ollama (default), vLLM, LM Studio, llama.cpp
- Cloud: OpenRouter, OpenAI, Anthropic, Groq, Gemini

Providers auto-registered from env vars. No separate skills per provider.
Cascade order configured via PROVIDER_CASCADE env var.

Also fix .env loading path in loop (was .local/share, now .config matches wizard).

harness/loop.org

@@ -139,7 +139,7 @@ The Metabolic Loop is the fundamental rhythm of OpenCortex: the continuous proce
 (defun main ()
   "Entry point for OpenCortex. Initializes the system and enters idle loop."
   (let* ((home (uiop:getenv "HOME"))
-         (env-file (uiop:merge-pathnames* ".local/share/opencortex/.env" (uiop:ensure-directory-pathname home))))
+         (env-file (uiop:merge-pathnames* ".config/opencortex/.env" (uiop:ensure-directory-pathname home))))
     (when (uiop:file-exists-p env-file)
       (cl-dotenv:load-env env-file)))