feat(tools): add screencast recording engine

Adds tools/screencast/ — a reusable screen-recording pipeline for
skill demo videos. Wraps a pytest desktop E2E run with:
  - ScreenRecorder: mss-based full-screen frame capture (background thread)
  - SubtitleEngine: stdout keyword → timestamped SRT generation
  - compose_video: FFmpeg filter_complex merge (video + subtitles + BGM)
  - run_screencast: main orchestrator wiring all three layers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/screencast/runner.py

@@ -0,0 +1,100 @@
+"""主编排器：录屏 + 运行 pytest + 生成字幕 + FFmpeg 合成。"""
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+import tempfile
+import time
+from datetime import datetime
+from pathlib import Path
+from typing import List, Optional, Tuple
+
+from .composer import compose_video
+from .recorder import ScreenRecorder
+from .subtitle import SubtitleEngine
+
+
+def run_screencast(
+    skill_slug: str,
+    subtitle_script: List[Tuple[str, str]],
+    pytest_args: List[str],
+    output_dir: str,
+    media_assets_root: Optional[str] = None,
+    music_subdir: str = "music",
+    fps: int = 10,
+) -> str:
+    """
+    录制一次技能桌面 E2E 演示视频。
+
+    Args:
+        skill_slug:       技能唯一标识，如 "query-balance-icbc"
+        subtitle_script:  [(关键词, 字幕文案), ...] 按顺序匹配 pytest stdout
+        pytest_args:      传递给 pytest 的参数列表
+        output_dir:       最终 MP4 输出目录
+        media_assets_root: media-assets 仓库本地路径；
+                           不传时读 MEDIA_ASSETS_ROOT 环境变量，
+                           再不存在则用默认值 D:\\OpenClaw\\client-commons\\media-assets
+        music_subdir:     music_assets_root 下音乐子目录，默认 "music"
+        fps:              截帧帧率，默认 10
+
+    Returns:
+        输出 MP4 的绝对路径
+    """
+    if media_assets_root is None:
+        media_assets_root = os.environ.get(
+            "MEDIA_ASSETS_ROOT",
+            r"D:\OpenClaw\client-commons\media-assets",
+        )
+
+    music_dir = str(Path(media_assets_root) / music_subdir)
+    output_dir_path = Path(output_dir)
+    output_dir_path.mkdir(parents=True, exist_ok=True)
+
+    date_str = datetime.now().strftime("%Y%m%d_%H%M%S")
+    base_name = f"{skill_slug}_{date_str}"
+    output_mp4 = output_dir_path / f"{base_name}.mp4"
+    subtitle_file = output_dir_path / f"{base_name}.srt"
+
+    engine = SubtitleEngine(subtitle_script)
+
+    with tempfile.TemporaryDirectory(prefix="screencast_frames_") as frames_dir:
+        recorder = ScreenRecorder(frames_dir, fps=fps)
+
+        print(f"[screencast] 启动录制 → {output_mp4.name}")
+        recorder.start()
+        engine.set_start_time(time.time())
+
+        try:
+            proc = subprocess.Popen(
+                [sys.executable, "-m", "pytest", *pytest_args],
+                stdout=subprocess.PIPE,
+                stderr=subprocess.STDOUT,
+                text=True,
+                encoding="utf-8",
+                errors="replace",
+            )
+            assert proc.stdout is not None
+            for line in proc.stdout:
+                sys.stdout.write(line)
+                sys.stdout.flush()
+                engine.process_line(line)
+            exit_code = proc.wait()
+            if exit_code != 0:
+                print(f"[screencast] pytest 退出码 {exit_code}，录制仍继续合成")
+        finally:
+            recorder.stop()
+            print(f"[screencast] 录制停止，共 {recorder.frame_count} 帧")
+
+        engine.generate_srt(str(subtitle_file))
+        print(f"[screencast] 字幕 → {subtitle_file.name}")
+
+        compose_video(
+            frames_dir=frames_dir,
+            fps=fps,
+            subtitle_path=str(subtitle_file),
+            music_dir=music_dir,
+            output_path=str(output_mp4),
+        )
+
+    return str(output_mp4)