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.schema.ts

@@ -35,6 +35,8 @@ export const rpcErrorSchema: z.ZodType<RpcError> = z.discriminatedUnion('code',
   z.object({ code: z.literal('bad-request'), message: z.string(), details: z.object({ issues: z.array(z.custom<ZodIssue>()) }) }),
   z.object({ code: z.literal('session-not-found'), message: z.string(), details: z.object({ sessionId: z.string() }) }),
   z.object({ code: z.literal('agent-busy'), message: z.string(), details: z.object({ reason: z.string() }) }),
+  z.object({ code: z.literal('command-error'), message: z.string(), details: z.object({}) }),
+  z.object({ code: z.literal('unknown-command'), message: z.string(), details: z.object({}) }),
   z.object({ code: z.literal('internal'), message: z.string(), details: z.object({}) }),
 ]) as unknown as z.ZodType<RpcError>
 

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': {}
 }
 

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. */

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

@@ -64,9 +64,16 @@ export interface SessionsApi {
   history(request: RpcRequest<{ sessionId: SessionId; beforeSeq?: number; maxMessages?: number }>):
   Promise<RpcResponse<{ events: HistoryEntry[]; hasMore: boolean }>>
 
-  /** Sends a message. content is core's ContentBlock[] verbatim; mode maps 1:1 — queue→send, steer→steer. */
+  /**
+   * Sends a message. content is core's ContentBlock[] verbatim; mode maps 1:1 — queue→send, steer→steer.
+   * A prompt whose content is exactly one text block starting with '/' is a slash command: the host
+   * executes it through the command registry (mode-agnostic) and it is never sent to the model. A
+   * successful command returns ok with the command slot (its success text, when the command produced
+   * one — carried for future rendering; the state change is the feedback). A usage/state error is an
+   * RPC error with code command-error; an unrecognized name is an RPC error with code unknown-command.
+   */
   prompt(request: RpcRequest<{ sessionId: SessionId; mode: 'queue' | 'steer'; content: ContentBlock[] }>):
-  Promise<RpcResponse<{ accepted: true }>>
+  Promise<RpcResponse<{ accepted: true; command?: { kind: 'success'; text?: string } }>>
 
   /** Stops: clears both FIFOs + aborts the current step (1:1 with agent.cancel). */
   cancel(request: RpcRequest<{ sessionId: SessionId }>): Promise<RpcResponse<{ accepted: true }>>