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/utils/oauth/load.ts

@@ -0,0 +1,21 @@
+import type { OAuthAuth } from "../../auth/types.ts";
+
+/**
+ * Loads an OAuth flow module through a variable specifier so bundlers cannot
+ * follow the import into Node-only flow code (`node:http` callback servers,
+ * `node:crypto` PKCE). The `.ts`/`.js` rewrite keeps the trick working from
+ * both source and built output.
+ */
+const importOAuthModule = (specifier: string): Promise<unknown> => {
+	const runtimeSpecifier = import.meta.url.endsWith(".js") ? specifier.replace(/\.ts$/, ".js") : specifier;
+	return import(runtimeSpecifier);
+};
+
+export const loadAnthropicOAuth = async (): Promise<OAuthAuth> =>
+	((await importOAuthModule("./anthropic.ts")) as { anthropicOAuth: OAuthAuth }).anthropicOAuth;
+
+export const loadOpenAICodexOAuth = async (): Promise<OAuthAuth> =>
+	((await importOAuthModule("./openai-codex.ts")) as { openaiCodexOAuth: OAuthAuth }).openaiCodexOAuth;
+
+export const loadGitHubCopilotOAuth = async (): Promise<OAuthAuth> =>
+	((await importOAuthModule("./github-copilot.ts")) as { githubCopilotOAuth: OAuthAuth }).githubCopilotOAuth;