feat(gui): host goal handlers and slash command dispatch on web prompts

- web host boots CommandService + command-goal; session.prompt intercepts
  a lone text block starting with '/' and executes it as a command
  (unknown -> RPC unknown-command, usage error -> RPC command-error, so
  the composer restores the draft and shows the error strip) instead of
  sending it to the model — symmetric with the ACP adapter
- prompt response carries command result text on the wire for future UI
- api-proxy also lands the host-side goals RPC handlers

packages/host/apiproxy/src/api/rpc.ts

@@ -32,6 +32,10 @@ export interface RpcErrorDetailsMap {
   'bad-request': { issues: ZodIssue[] }
   'session-not-found': { sessionId: SessionId }
   'agent-busy': { reason: string }
+  /** A known slash command reported a usage/state error; the message is the command's own text. */
+  'command-error': {}
+  /** A leading-/ prompt named no registered command; the message names the token. */
+  'unknown-command': {}
   'internal': {}
 }