feat: sync running tasks over WebSocket and drop the REST task API

.changeset/remove-rest-task-api.md

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+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Remove the REST task endpoints (`GET /sessions/{id}/tasks`, `GET /sessions/{id}/tasks/{task_id}`, and the `:cancel` / `:detach` actions); use the WebSocket task frames and the `task.stop` / `task.background` requests instead.

.changeset/task-output-watch.md

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+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Stream a task's output over the server's WebSocket with `task.watch` / `task.unwatch`, and stop a task or move it to the background with `task.stop` / `task.background`.

.changeset/task-ws-events.md

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+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Send each agent's running tasks over the server WebSocket as a `task.list` frame followed by a `task` frame on every change; transcripts and the REST transcript endpoints no longer include tasks.

.changeset/web-interaction-polish.md

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+---
+"@moonshot-ai/kimi-code": patch
+---
+
+web: Improved interactions and fixed known bugs.