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).

opencortex.sh

@@ -29,7 +29,9 @@ export MEMEX_DIR="${MEMEX_DIR:-$HOME/memex}"
 
 # Load environment variables from the standard config location
 if [ -f "$OC_CONFIG_DIR/.env" ]; then
+    set -a
     source "$OC_CONFIG_DIR/.env"
+    set +a
 fi
 
 # --- Dependency Checker ---