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/apiproxy/src/invariant.ts

@@ -15,11 +15,12 @@ export const name = 'host-apiproxy-invariant'
 export const inject = ['invariants']
 
 /**
- * No runtime invariant: this package is the wire contract layer (types,
- * schemas, fetch carrier glue) — it emits no cordis events and owns no
- * mutable cross-plugin relation. rpcId round-trip and schema acceptance are
- * enforced at the carrier boundary and exercised by the protocol-isomorphism
- * suite; the live implementation relations belong to dsh-host-runtime.
+ * No runtime invariant: this package is the wire contract layer plus the
+ * host-side gateway over services owned elsewhere — it emits no cordis events
+ * of its own; the session/agent event streams it projects are asserted by
+ * their owning packages' companions. rpcId round-trip and schema acceptance
+ * are enforced at the carrier boundary and exercised by the
+ * protocol-isomorphism suite.
  */
 const install: InvariantInstaller = () => {}