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/antigravity.js

@@ -0,0 +1,27 @@
+import { platform, arch } from "os";
+
+export default {
+  id: "antigravity",
+  alias: "ag",
+  transport: {
+    baseUrls: [
+      "https://daily-cloudcode-pa.googleapis.com",
+      "https://daily-cloudcode-pa.sandbox.googleapis.com"
+    ],
+    format: "antigravity",
+    headers: { "User-Agent": `antigravity/1.107.0 ${platform()}/${arch()}` },
+    clientId: "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com",
+    clientSecret: "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"
+  },
+  models: [
+    { id: "gemini-3-flash-agent", name: "Gemini 3.5 Flash (High)" },
+    { id: "gemini-3.5-flash-low", name: "Gemini 3.5 Flash (Medium)" },
+    { id: "gemini-3.5-flash-extra-low", name: "Gemini 3.5 Flash (Low)" },
+    { id: "gemini-pro-agent", name: "Gemini 3.1 Pro (High)" },
+    { id: "gemini-3.1-pro-low", name: "Gemini 3.1 Pro (Low)" },
+    { id: "claude-sonnet-4-6", name: "Claude Sonnet 4.6 (Thinking)" },
+    { id: "claude-opus-4-6-thinking", name: "Claude Opus 4.6 (Thinking)" },
+    { id: "gpt-oss-120b-medium", name: "GPT-OSS 120B (Medium)" },
+    { id: "gemini-3-flash", name: "Gemini 3 Flash", thinking: false }
+  ]
+};