fix(ai,coding-agent): make pi-ai browser-safe and move OAuth runtime exports

- add browser smoke bundling check to root check + pre-commit

- lazy-load Bedrock provider registration to avoid browser graph traversal

- remove top-level OAuth runtime exports from @mariozechner/pi-ai

- add @mariozechner/pi-ai/oauth subpath export and update coding-agent imports

- move proxy dispatcher init to coding-agent CLI (Node-only)

- document Bedrock/OAuth browser limitations

closes #1814

scripts/browser-smoke-entry.ts

@@ -0,0 +1,4 @@
+import { complete, getModel } from "@mariozechner/pi-ai";
+
+const model = getModel("google", "gemini-2.5-flash");
+console.log(model.id, typeof complete);