feat(core,server,web,cli): mid-run steering messages and square-bracket markers (#63)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/web/src/api/endpoints.ts

@@ -51,6 +51,7 @@ import type {
   SessionTracesResponse,
   SkillInstallRequest,
   SkillLibraryResponse,
+  SteerRequest,
   TaskCreateRequest,
   TaskCreateResponse,
   TraceAnalysisResponse,
@@ -264,6 +265,13 @@ export const postAbort = (sessionId: string) =>
     body: {},
   });
 
+/** Mid-run steering: queues a message for the running Task (delivered between turns as a standalone `[user_steering]` user message); 409 not_running when no Task is in progress. */
+export const postSteer = (sessionId: string, body: SteerRequest) =>
+  apiFetch<void>(`/api/sessions/${encodeURIComponent(sessionId)}/steer`, {
+    method: "POST",
+    body,
+  });
+
 export const postCompact = (sessionId: string) =>
   // Same shape as tasks: the response carries the actual current session_id (a new id after self-healing; the frontend updates its route accordingly).
   apiFetch<TaskCreateResponse>(`/api/sessions/${encodeURIComponent(sessionId)}/compact`, {