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/anthropic.js

@@ -0,0 +1,16 @@
+import { CLAUDE_API_HEADERS } from "../shared.js";
+
+export default {
+  id: "anthropic",
+  alias: "anthropic",
+  transport: {
+    baseUrl: "https://api.anthropic.com/v1/messages",
+    format: "claude",
+    headers: { ...CLAUDE_API_HEADERS }
+  },
+  models: [
+    { id: "claude-sonnet-4-20250514", name: "Claude Sonnet 4" },
+    { id: "claude-opus-4-20250514", name: "Claude Opus 4" },
+    { id: "claude-3-5-sonnet-20241022", name: "Claude 3.5 Sonnet" }
+  ]
+};