feat(ai): move API implementations to src/api with lazy wrappers (phase 2)

Stream implementations move from src/providers/ to src/api/, renamed by
API id (anthropic.ts -> anthropic-messages.ts, google.ts ->
google-generative-ai.ts, mistral.ts -> mistral-conversations.ts,
amazon-bedrock.ts -> bedrock-converse-stream.ts). Every module now
exports exactly stream/streamSimple; shared helpers move alongside.

New ProviderStreams dispatch contract in types.ts, lazyApi() wrapper in
api/lazy.ts, and one .lazy.ts wrapper per API. Bedrock's wrapper keeps
the node-only variable-specifier import and setBedrockProviderModule()
(now taking ProviderStreams).

providers/register-builtins.ts deleted; interim until the compat
entrypoint lands, builtin api-registry registration lives in stream.ts
and lazy wrappers are exported from the root barrel. Old per-API lazy
exports (streamAnthropic, ...) are gone; package.json subpaths retarget
to dist/api/.

packages/ai/test/openai-codex-stream.test.ts

@@ -5,9 +5,9 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import {
 	getOpenAICodexWebSocketDebugStats,
 	resetOpenAICodexWebSocketDebugStats,
-	streamOpenAICodexResponses,
-	streamSimpleOpenAICodexResponses,
-} from "../src/providers/openai-codex-responses.ts";
+	stream as streamOpenAICodexResponses,
+	streamSimple as streamSimpleOpenAICodexResponses,
+} from "../src/api/openai-codex-responses.ts";
 import type { Context, Model } from "../src/types.ts";
 
 const originalAgentDir = process.env.PI_CODING_AGENT_DIR;