refactor(web): upgrade apiproxy to the api-gateway service plugin

createApiProxy moves from dsh-host-runtime into dsh-host-apiproxy (the
dependency direction already pointed this way); the package now
default-exports ApiProxyService (config {provider, model}, provides
ctx.apiProxy) while staying transport-agnostic — it registers no routes.
Runtime keeps bootHost/startHost for the headless path with its import
re-anchored, and drops the mountWebPlugins roster mounting helper.

packages/host/runtime/tests/api-proxy-view.spec.ts

@@ -21,7 +21,7 @@ import type { ToolDefinition } from '@deepseek-ai/dsh-tools'
 import UserInteractionService from '@deepseek-ai/dsh-user-interaction'
 import type { MuxFrame, RpcRequest } from '@deepseek-ai/dsh-host-apiproxy/api'
 import { RpcId } from '@deepseek-ai/dsh-host-apiproxy/api/rpc'
-import { createApiProxy } from '../src/api-proxy.ts'
+import { createApiProxy } from '@deepseek-ai/dsh-host-apiproxy'
 
 const reply = (text: string): Promise<ContentBlock[]> => Promise.resolve([{ type: 'text', text }])