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)

docs/ROADMAP.org

@@ -211,7 +211,7 @@ CLOSED: [2026-05-03 Sun 10:42]
 - State "DONE" from "TODO" [2026-05-03 Sun 10:42]
 :END:
 =vault-get-secret= and =vault-set-secret= are exported from =core-defpackage=
-and called from =gateway-manager.org= (lines 36, 86, 180) but never defined.
+and called from =gateway-messaging.org= (lines 36, 86, 180) but never defined.
 =gateway-link= crashes at runtime. Add one-line wrappers in =security-vault.org=
 that delegate to the existing =vault-get=/=vault-set= with ~:type :secret~.