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)

org/system-model.org

@@ -0,0 +1,39 @@
+#+TITLE: SKILL: Model Dispatch (org-skill-model.org)
+#+AUTHOR: Agent
+#+FILETAGS: :skill:model:dispatch:
+#+PROPERTY: header-args:lisp :tangle ../lisp/system-model.lisp
+
+* Architectural Intent
+
+~system-model~ is the dispatcher. It sits between the reason pipeline and the provider backends registered in ~*probabilistic-backends*~. The reason pipeline calls ~model-request~ with a provider keyword and a model name; ~model-request~ looks up that provider's handler function, calls it, and returns the result.
+
+This is intentionally thin. All the provider-specific logic (URL construction, API key headers, JSON parsing) lives in ~system-model-provider~. All the routing logic (which model for which task) lives in ~system-model-router~. This skill is the seam — it decouples "who to call" from "how to call them" and "why to call them."
+
+When every provider fails (not registered, or all return errors), ~model-request~ returns an error plist with ~:status :error~. The reason pipeline's ~backend-cascade-call~ catches this and falls through to the next provider in the cascade. If no provider can serve the request, the cascade returns the "Neural Cascade Failure" signal.
+
+~model-request~ replaces the old ~gateway-llm-request~ with the same contract but renamed for consistency with the ~system-model-*~ family.
+
+* Implementation
+
+** Request execution
+#+begin_src lisp
+(in-package :passepartout)
+
+(defun model-request (&key prompt system-prompt (provider :openrouter) model)
+  "Central dispatcher for LLM requests."
+  (let ((backend (gethash provider *probabilistic-backends*)))
+    (if backend
+        (handler-case
+            (funcall backend prompt system-prompt :model model)
+          (error (c)
+            (list :status :error :message (format nil "~a Failure: ~a" provider c))))
+        (list :status :error :message (format nil "Provider ~a not registered" provider)))))
+#+end_src
+
+** Skill registration
+#+begin_src lisp
+(defskill :passepartout-system-model
+  :priority 100
+  :trigger (lambda (ctx) (getf ctx :user-input))
+  :deterministic (lambda (action ctx) (declare (ignore ctx)) action))
+#+end_src