refactor(open-sse): P3 provider registry — split into providers/registry/{id}.js

- 100 registry files: transport + models co-located per provider (Mongoose-style)
- providers/shared.js: shared Claude headers + OS/arch helpers (deduped)
- providers/models/helpers.js: withCodexReviewModels (kept dynamic)
- providers/index.js builds PROVIDERS + PROVIDER_MODELS = old API (content byte-for-byte)
- config/providers.js + providerModels.js → barrel re-export, keep accessors + runtime helpers
- Verified: PROVIDERS/MODELS/OAuth/alias byte-for-byte, golden translator tests pass, 0 new regression

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/providers/registry/mistral.js

@@ -0,0 +1,26 @@
+export default {
+  "id": "mistral",
+  "alias": "mistral",
+  "transport": {
+    "baseUrl": "https://api.mistral.ai/v1/chat/completions"
+  },
+  "models": [
+    {
+      "id": "mistral-large-latest",
+      "name": "Mistral Large 3"
+    },
+    {
+      "id": "codestral-latest",
+      "name": "Codestral"
+    },
+    {
+      "id": "mistral-medium-latest",
+      "name": "Mistral Medium 3"
+    },
+    {
+      "id": "mistral-embed",
+      "name": "Mistral Embed",
+      "type": "embedding"
+    }
+  ]
+};