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/sessions.schema.ts

@@ -94,9 +94,13 @@ export const sessionPromptRequestSchema = z.object({
   content: z.array(contentBlockSchema),
 }) as unknown as z.ZodType<RequestPayload<'session.prompt'>>
 
-/** session.prompt response value. */
+/** session.prompt response value (the command slot appears only when the prompt dispatched a slash command). */
 export const sessionPromptValueSchema = z.object({
   accepted: z.literal(true),
+  command: z.object({
+    kind: z.literal('success'),
+    text: z.string().optional(),
+  }).optional(),
 }) satisfies z.ZodType<Wire<ResponseValue<'session.prompt'>>>
 
 /** session.cancel request payload. */