feat: preserve multi-track audio in clip extraction

SKILL.md:
- Step 6: two clip modes — video-only (frame extraction) vs all-streams (editor use)
- Pitfall #10: warn about dropping audio tracks with default -map 0:v:0
- Note on keyframe accuracy vs frame-exact re-encode

extract_frames.py:
- New --all-streams flag on clip subcommand
- -map 0 preserves video + all audio tracks + subtitles
- Default remains -map 0:v:0 for fast frame extraction

scripts/frames/extract_frames.py

@@ -65,13 +65,19 @@ def cmd_clip(args: argparse.Namespace) -> None:
     if not video.exists():
         sys.exit(f"Video not found: {video}")
 
+    # Build stream mapping: video-only for frame extraction, or all streams for editor use
+    if args.all_streams:
+        stream_map = ["-map", "0"]  # all streams (video + all audio + subtitles)
+    else:
+        stream_map = ["-map", "0:v:0"]  # video only (faster, smaller for frame extraction)
+
     cmd = [
         "ffmpeg", "-hide_banner", "-y",
         "-ss", str(args.start),
         "-to", str(args.end),
         "-i", str(video),
         "-c", "copy",
-        "-map", "0:v:0",
+        *stream_map,
         str(output),
     ]
 
@@ -192,6 +198,8 @@ def main() -> None:
     p_clip.add_argument("--start", required=True, type=float, help="Start time in seconds")
     p_clip.add_argument("--end", required=True, type=float, help="End time in seconds")
     p_clip.add_argument("--output", required=True, help="Output clip path")
+    p_clip.add_argument("--all-streams", action="store_true", default=False,
+                        help="Preserve all streams (video + all audio tracks + subtitles). Default: video only (for frame extraction)")
 
     # frames
     p_frames = sub.add_parser("frames", help="Extract frames from a video/clip")