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/coding-agent/examples/extensions/custom-provider-anthropic/index.ts

@@ -153,7 +153,7 @@ async function refreshAnthropicToken(credentials: OAuthCredentials): Promise<OAu
 }
 
 // =============================================================================
-// Streaming Implementation (simplified from packages/ai/src/providers/anthropic.ts)
+// Streaming Implementation (simplified from packages/ai/src/api/anthropic-messages.ts)
 // =============================================================================
 
 // Claude Code tool names for OAuth stealth mode

packages/coding-agent/examples/extensions/custom-provider-gitlab-duo/index.ts

@@ -12,14 +12,14 @@
 import {
 	type Api,
 	type AssistantMessageEventStream,
+	anthropicMessagesApi,
 	type Context,
 	createAssistantMessageEventStream,
 	type Model,
 	type OAuthCredentials,
 	type OAuthLoginCallbacks,
+	openAIResponsesApi,
 	type SimpleStreamOptions,
-	streamSimpleAnthropic,
-	streamSimpleOpenAIResponses,
 	type ThinkingLevelMap,
 } from "@earendil-works/pi-ai";
 import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
@@ -325,7 +325,7 @@ export function streamGitLabDuo(
 
 			const innerStream =
 				cfg.backend === "anthropic"
-					? streamSimpleAnthropic(
+					? anthropicMessagesApi().streamSimple(
 							{
 								...(modelWithBaseUrl as Model<"anthropic-messages">),
 								compat: {
@@ -336,7 +336,11 @@ export function streamGitLabDuo(
 							context,
 							streamOptions,
 						)
-					: streamSimpleOpenAIResponses(modelWithBaseUrl as Model<"openai-responses">, context, streamOptions);
+					: openAIResponsesApi().streamSimple(
+							modelWithBaseUrl as Model<"openai-responses">,
+							context,
+							streamOptions,
+						);
 
 			for await (const event of innerStream) stream.push(event);
 			stream.end();