Add comprehensive existing file handling documentation to Create-Clip.md, Create-PillarBox.md, Create-Turnaround.md and Get-LastFrame.md explaining ffmpeg -y replacement behavior and WhatIf safety recommendations, expand SKILL.md prerequisites section to clarify Windows 11-only platform support with pwsh.exe requirement and reorganize core vs optional tools with task-specific check guidance, update conventions section to add Frames/Frame syntax tables and .skip marker behavior details, replace safety

skill/Create-PillarBox.md

@@ -218,6 +218,13 @@ The same logic applies to a transposed 9:16 frame:
 # CRF 18 instead of default CRF 8 / source bitrate
 ```
 
+## Existing Files
+
+An existing `-Target` video is **replaced** without a prompt. ffmpeg runs with `-y`.
+
+- If ffmpeg fails, the target file is removed and the script stops with an error. This also removes a target that existed before the run, so the old file is lost. A partial file is not left behind.
+- Use `-WhatIf` first, or write to a new name.
+
 ## Safe Execution
 
 This script supports `-WhatIf` and `-Confirm` for safe execution: