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/agent/test/agent.test.ts

@@ -1,4 +1,4 @@
-import { type AssistantMessage, type AssistantMessageEvent, EventStream, getModel } from "@earendil-works/pi-ai";
+import { type AssistantMessage, type AssistantMessageEvent, EventStream, getModel } from "@earendil-works/pi-ai/compat";
 import { describe, expect, it } from "vitest";
 import { Agent } from "../src/index.ts";
 

packages/agent/test/harness/agent-harness.test.ts

@@ -1,4 +1,4 @@
-import { fauxAssistantMessage, fauxToolCall, getModel, registerFauxProvider } from "@earendil-works/pi-ai";
+import { fauxAssistantMessage, fauxToolCall, getModel, registerFauxProvider } from "@earendil-works/pi-ai/compat";
 import { afterEach, describe, expect, it } from "vitest";
 import { AgentHarness } from "../../src/harness/agent-harness.ts";
 import { NodeExecutionEnv } from "../../src/harness/env/nodejs.ts";

packages/agent/test/scratch/simple.ts

@@ -1,6 +1,6 @@
 import { homedir } from "node:os";
 import { join } from "node:path";
-import { getModel } from "@earendil-works/pi-ai";
+import { getModel } from "@earendil-works/pi-ai/compat";
 import { NodeExecutionEnv } from "../../src/harness/env/nodejs.ts";
 import { InMemorySessionStorage } from "../../src/harness/session/memory-storage.ts";
 import {