feat: add video recording capability (bsk video command) / 给 bsk 增加录制视频的能力

English

What

Give bsk the ability to record video of a browser session — a new bsk video capability that captures the Agent Window / session tab as a playable video file.

Scenarios

• Automated patrols / walkthroughs (自动化巡查): when a Skill runs a patrol, the user gets a walkthrough video of what the agent saw and did, page by page — instead of reconstructing it from text logs or static screenshots.
• Reviewing agent behaviour after the fact, sharing run recordings with teammates, debugging failed automations.

Current state

• bsk record captures the user's own actions into a trace.json for replay — not a video.
• bsk screenshot produces stills (and since #64 they exclude BSK overlays).
• There is no way to get a continuous visual record of an agent session.

Requirements (decided)

bsk video start --session <id> [--max-duration 5m]
bsk video stop --out /path/to/walkthrough.mp4

1. No recording indicator — do NOT use chrome.tabCapture (Chrome always shows its recording banner). Capture frames via CDP Page.startScreencast / screencastFrame instead.
2. MP4 output — encode the captured frames to MP4.
3. Output path comes from a parameter (--out); no default directory.
4. Hide overlays during recording — reuse the hide-during-capture mechanism from #64 for the recording duration.
5. Duration cap: default 1 minute, configurable (e.g. --max-duration); hitting the cap auto-stops cleanly and still produces a valid file.

Acceptance criteria

• Start/stop recording of a session tab from the CLI; produces a playable MP4 of the walkthrough.
• Recording survives navigations within the tab; clear error when the session/tab is gone.
• No recording banner, no overlay artifacts in the video.
• Duration cap honored (default 60s, configurable) with clean finalization.
• Docs in SKILL.md on how agents use it during patrols.

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中文

是什么

给 bsk 增加录制视频的能力——新增 bsk video 命令，把 Agent Window / 会话 tab 的操作过程录成可播放的视频文件。

场景

• 自动化巡查：用 Skill 做巡查时，把走查过程录成视频给用户——agent 逐页看了什么、做了什么——而不是靠文字日志或静态截图还原。
• 事后回看 agent 行为、把运行录像分享给同事、调试失败的自动化任务。

现状

• bsk record 录的是用户操作轨迹（trace.json，用于回放），不是视频。
• bsk screenshot 是静态截图（#64 起已排除 BSK Overlay）。
• 没有任何办法拿到 agent 会话的连续影像。

需求（已定）

bsk video start --session <id> [--max-duration 5m]
bsk video stop --out /path/to/walkthrough.mp4

1. 不要录制指示条：不用 chrome.tabCapture（Chrome 必显示录制横幅），改走 CDP Page.startScreencast / screencastFrame 取帧。
2. 输出 MP4：把取到的帧编码成 MP4。
3. 文件位置由传入参数决定（--out），不设默认目录。
4. 录制期间隐藏 Overlay：复用 #64 的截图隐藏机制。
5. 时长上限默认 1 分钟、可通过配置修改（如 --max-duration），到上限自动正常收尾、产出完整文件。

验收标准

• CLI 可对会话 tab 开始/结束录制，产出可播放的 MP4 走查视频。
• 录制跨页面导航不中断；会话/tab 消失时报错清晰。
• 视频无录制横幅、无 Overlay 杂物。
• 时长上限生效（默认 60 秒、可配置），截断后文件仍完整可播。
• SKILL.md 补充 agent 巡查时如何使用。

Start with the existing bsk record and bsk screenshot entry points, then trace how session tabs and the #64 overlay-hiding mechanism are handled. Read the CDP Page.startScreencast and screencastFrame flow before designing bsk video start/stop and MP4 finalization. Done means the acceptance criteria pass, including navigation, duration caps, clean errors, and the SKILL.md patrol instructions.