feat: add SDK foundation layer

Implement config resolution, typed errors, HTTP and WebSocket transport
helpers, and timeout handling for the SDK foundation. Add unit and local
integration tests covering the SDK foundation behaviour and align
package exports with the tsup output.

tsup.config.ts

@@ -2,10 +2,13 @@ import { defineConfig } from "tsup";
 
 export default defineConfig({
 	entry: ["src/index.ts"],
-	format: ["cjs", "esm"],
-	dts: { resolve: true },
+	format: ["esm", "cjs"],
+	outExtension({ format }) {
+		return { js: format === "esm" ? ".js" : ".cjs" };
+	},
 	outDir: "dist",
 	clean: true,
 	sourcemap: true,
 	minify: false,
+	dts: { resolve: true },
 });