feat(ai): compat entrypoint, core-only root barrel (phase 5)

The root barrel is now core-only and side-effect free: types,
createModels/createProvider, auth substrate, lazyStream/lazyApi, faux,
utils. Generated catalogs, api-registry, env-api-keys, images, global
stream functions, and per-API lazy wrappers leave the root.

New @earendil-works/pi-ai/compat preserves the old surface verbatim as
a strict superset of the root: api-dispatch stream/complete with env
key injection, the builtin registration side effect (skip-if-present so
it cannot clobber earlier overrides), deprecated getModel/getModels/
getProviders aliases of the new getBuiltin* reads in providers/all,
lazy api wrappers + setBedrockProviderModule, and image generation.
Compat dies with the coding-agent ModelManager migration.

Packaging: exports map gains ./compat, ./providers/*, ./api/*;
sideEffects array lists only the effectful modules.

Old-global imports across agent/coding-agent/examples and pi-ai tests
switch to /compat (path-only; compat is a superset). The coding-agent
extension loader resolves the pi-ai ROOT specifier to compat, so
existing user extensions using the old global API keep working at
runtime until compat is removed. vitest configs alias /compat to src;
browser smoke imports old globals from /compat.

packages/ai/src/utils/oauth/github-copilot.ts

@@ -3,7 +3,7 @@
  */
 
 import type { OAuthAuth, OAuthCredential } from "../../auth/types.ts";
-import { getModels } from "../../models.ts";
+import { GITHUB_COPILOT_MODELS } from "../../providers/github-copilot.models.ts";
 import type { Api, Model } from "../../types.ts";
 import { pollOAuthDeviceCodeFlow } from "./device-code.ts";
 import type { OAuthCredentials, OAuthDeviceCodeInfo, OAuthLoginCallbacks, OAuthProviderInterface } from "./types.ts";
@@ -274,7 +274,7 @@ async function enableAllGitHubCopilotModels(
 	enterpriseDomain?: string,
 	onProgress?: (model: string, success: boolean) => void,
 ): Promise<void> {
-	const models = getModels("github-copilot");
+	const models = Object.values(GITHUB_COPILOT_MODELS);
 	await Promise.all(
 		models.map(async (model) => {
 			const success = await enableGitHubCopilotModel(token, model.id, enterpriseDomain);