feat(ai): add Models runtime with provider-owned auth (phase 1)

New Models/MutableModels/createModels collection: provider map, async
model listing (best-effort aggregation), getAuth decision tree with
double-checked locked OAuth refresh, stream/complete with per-field
auth merge over lazyStream.

Auth substrate: ProviderAuth { apiKey?, oauth? }, one type-tagged
credential per provider, CredentialStore (read/modify/delete; modify
is the only write path, serialized RMW), OAuthAuth login/refresh/toAuth
split, prompt()/notify() login callbacks, browser-safe default
AuthContext.

types.ts: Provider alias renamed to ProviderId; ApiOptionsMap and
ApiStreamOptions<TApi> for typed per-API stream options; hasApi()
runtime narrowing guard.

packages/ai/src/auth/context.ts

@@ -0,0 +1,45 @@
+import type { AuthContext } from "./types.ts";
+
+interface NodeFsModule {
+	access(path: string): Promise<void>;
+}
+
+interface NodeOsModule {
+	homedir(): string;
+}
+
+// Variable specifier so browser bundlers do not try to resolve node builtins.
+const importNodeModule = (specifier: string): Promise<unknown> => import(specifier);
+
+function getProcessEnv(): Record<string, string | undefined> | undefined {
+	const proc = (globalThis as { process?: { env?: Record<string, string | undefined> } }).process;
+	return proc?.env;
+}
+
+/**
+ * Default auth context: env vars from `process.env` (undefined in browsers),
+ * file existence via node:fs (always false in browsers).
+ */
+export function defaultProviderAuthContext(): AuthContext {
+	return {
+		async env(name: string): Promise<string | undefined> {
+			const value = getProcessEnv()?.[name];
+			return typeof value === "string" && value.trim().length > 0 ? value : undefined;
+		},
+
+		async fileExists(path: string): Promise<boolean> {
+			try {
+				const fs = (await importNodeModule("node:fs/promises")) as NodeFsModule;
+				let resolved = path;
+				if (resolved.startsWith("~")) {
+					const os = (await importNodeModule("node:os")) as NodeOsModule;
+					resolved = os.homedir() + resolved.slice(1);
+				}
+				await fs.access(resolved);
+				return true;
+			} catch {
+				return false;
+			}
+		},
+	};
+}