feat(ai): provider factories, per-provider catalogs, createProvider (phase 3)

Auth helpers in src/auth/helpers.ts: envApiKeyAuth() (stored key wins,
then env vars in order, with secret-prompt login) and lazyOAuth()
(flow loads on first use through bundler-opaque dynamic imports in
utils/oauth/load.ts; the OAuthAuth flow exports land in phase 4).
There is no OAuth factory toggle: providers that support OAuth always
attach it, advertising costs nothing until login/refresh runs.

createProvider() in models.ts builds providers from parts: single API
implementation or a map dispatched on model.api (mixed-API providers
like opencode and github-copilot); unknown api yields a stream error.

generate-models.ts now emits one providers/<id>.models.ts catalog per
provider (35 files, biome-excluded like models.generated.ts) and
models.generated.ts becomes a generated aggregator, so importing one
provider factory pulls one catalog. Typed getModel globals unchanged.

One factory per built-in provider under src/providers/: envApiKeyAuth
for standard providers, OAuth for anthropic/openai-codex/github-copilot,
ambient ApiKeyAuth for amazon-bedrock (AWS env/profile/IAM) and
google-vertex (explicit key or ADC+project+location).

providers/all.ts: builtinProviders(), builtinModels(), getBuiltin*
re-exports. fauxProvider() factory returns a real Provider for tests;
legacy registerFauxProvider() unchanged.

packages/ai/src/providers/google-vertex.ts

@@ -0,0 +1,38 @@
+import { googleVertexApi } from "../api/google-vertex.lazy.ts";
+import type { ApiKeyAuth } from "../auth/types.ts";
+import { createProvider, type Provider } from "../models.ts";
+import { GOOGLE_VERTEX_MODELS } from "./google-vertex.models.ts";
+
+const VERTEX_ADC_PATH = "~/.config/gcloud/application_default_credentials.json";
+
+/**
+ * Vertex accepts an explicit API key or Application Default Credentials
+ * (`gcloud auth application-default login`). ADC additionally requires
+ * project and location env vars, which the implementation reads itself.
+ */
+const vertexAuth: ApiKeyAuth = {
+	name: "Google Cloud credentials",
+	resolve: async ({ ctx, credential }) => {
+		const key = credential?.key ?? (await ctx.env("GOOGLE_CLOUD_API_KEY"));
+		if (key) return { auth: { apiKey: key }, source: credential?.key ? "stored credential" : "GOOGLE_CLOUD_API_KEY" };
+
+		const adcPath = await ctx.env("GOOGLE_APPLICATION_CREDENTIALS");
+		const hasCredentials = await ctx.fileExists(adcPath ?? VERTEX_ADC_PATH);
+		const hasProject = Boolean((await ctx.env("GOOGLE_CLOUD_PROJECT")) ?? (await ctx.env("GCLOUD_PROJECT")));
+		const hasLocation = Boolean(await ctx.env("GOOGLE_CLOUD_LOCATION"));
+		if (hasCredentials && hasProject && hasLocation) {
+			return { auth: {}, source: "gcloud application default credentials" };
+		}
+		return undefined;
+	},
+};
+
+export function googleVertexProvider(): Provider<"google-vertex"> {
+	return createProvider({
+		id: "google-vertex",
+		name: "Google Vertex AI",
+		auth: { apiKey: vertexAuth },
+		models: Object.values(GOOGLE_VERTEX_MODELS),
+		api: googleVertexApi(),
+	});
+}