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/src/start.ts

@@ -8,10 +8,9 @@
 
 import type { Context } from 'cordis'
 import type { ApiProxy } from '@deepseek-ai/dsh-host-apiproxy/api'
-import { toFetchHandler } from '@deepseek-ai/dsh-host-apiproxy'
+import { createApiProxy, toFetchHandler } from '@deepseek-ai/dsh-host-apiproxy'
 import { bootHost } from './boot.ts'
 import type { BootHostOptions, HostDefaults } from './boot.ts'
-import { createApiProxy } from './api-proxy.ts'
 
 /** Options for startHost. */
 export interface StartHostOptions {