Merge upstream/main into codex/btw-side-question

同步官方 main 的任务会话与 spawn_task 更新，保留 /btw 旁路上下文能力。

web/src/lib/api.ts

@@ -678,6 +678,16 @@ export const api = {
     }));
   },
 
+  // Main-agent conversation segments of a task. Each segment is a resettable session
+  // (clean transcript/context) over the same task; `current` is the writable one.
+  mainSessions: (task: string) =>
+    get<{ sessions: { seq: number; created_at: string }[]; current: number }>(
+      `/exploration/main-sessions${tq(task)}`,
+    ).then((r) => ({ sessions: arr(r.sessions), current: r.current ?? 0 })),
+  // Start a fresh main-agent session segment (does not touch the task's graph/assets/goal).
+  newMainSession: (task: string) =>
+    post<{ seq: number; created_at: string; current: number }>(`/exploration/main-session/new${tq(task)}`),
+
   // ---- traffic / audit / report / chat ----
   audit: (task?: string) => get<Audit>(`/audit${tq(task)}`),
   traffic: (page = 0, size = 100, host = "", method = "", q = "") =>
@@ -711,8 +721,8 @@ export const api = {
     if (!r.ok) throw new Error(`report: ${r.status}`);
     return r.text();
   },
-  chat: (message: string, task?: string, attachments?: ChatAttachment[]) =>
-    post<{ reply: string; mode: string }>(`/chat${tq(task)}`, { message, attachments }),
+  chat: (message: string, task?: string, attachments?: ChatAttachment[], seg?: number) =>
+    post<{ reply: string; mode: string }>(`/chat${tq(task)}`, { message, attachments, seg }),
   chatStatus: (taskId: string) => get<{ running: boolean }>(`/tasks/${taskId}/chat/status`),
   // 方式1 文件上传:落到会话/任务工作目录 uploads/，返回可供 agent Read 的相对路径。
   chatUpload: async (scope: "task" | "session" | "staging", id: string, files: File[]) => {

web/src/lib/types.ts

@@ -627,6 +627,7 @@ export interface Activity {
   };
   source_task_id?: string;
   inherited?: boolean;
+  main_seg?: number; // main-agent conversation segment (present only on worker="mainagent" rows)
   // token usage (present only on kind='result')
   input_tokens?: number;
   output_tokens?: number;
@@ -829,6 +830,7 @@ export interface Session {
   intent_id?: string;
   source_task_id?: string;
   inherited?: boolean;
+  seg?: number; // main-agent session: which conversation segment (0 = original)
 }
 
 // ---- Security ----