rename gateway-* → system-model-* + gateway-messaging, de-ollama, add system-model-explorer

- Rename gateway-provider → system-model-provider (generic :local provider, no hardcoded ollama)
- Rename gateway-llm → system-model (model-request dispatcher)
- Rename system-embedding-gateway → system-model-embedding
- Rename gateway-manager → gateway-messaging (public api renamed to messaging-*)
- Add system-model-explorer (model discovery via OpenRouter API, cached, per-slot recommendations)
- Fix skill loader export: replace prefix-matching with fbound/boundp-based export (20 skills now export)
- Add model-router to skill-loader exclusion list (loaded via CLI)
- De-ollama: remove hardcoded assumed-available patterns from provider pipeline
- Default cascade: cloud-only (openrouter, openai, groq, gemini, deepseek, nvidia, anthropic)
- Env example: add LOCAL_BASE_URL, fix cascade order
- All org files updated with architectural prose (literate programming)

passepartout.asd

@@ -30,7 +30,8 @@
                (:file "tests/tui-tests")
                (:file "tests/utils-org-tests")
                (:file "tests/utils-lisp-tests")
-               (:file "tests/llm-gateway-tests")))
+               (:file "tests/llm-gateway-tests")
+               (:file "tests/model-explorer-tests")))
 
 (defsystem :passepartout/tui
   :depends-on (:passepartout :croatoan :usocket :bordeaux-threads)