[BUG] Write tool shows "Error writing file" with relative paths but files are written successfully

Bug description

Write tool reports "Error writing file" when relative paths are used, but the files are actually created correctly on disk. The UI shows red "Error writing file" for each file, which is confusing since there's no actual error.

Steps to reproduce

1. Agent uses Write tool with a relative path (e.g. app/Mcp/Tools/GetMyTasksTool.php) instead of an absolute path
2. File is written successfully to disk
3. UI shows "Error writing file" in red for every file

Screenshot

Multiple files written in a batch all show the error:

● Write(app/Mcp/Tools/GetMyTasksTool.php)
  ⎿ Error writing file

● Write(app/Mcp/Tools/GetTaskByIdTool.php)
  ⎿ Error writing file

● Write(app/Mcp/Tools/CreateTaskTool.php)
  ⎿ Error writing file

All files were created correctly on disk with the expected content.

Expected behavior

• Either: no error (resolve relative path to absolute automatically)
• Or: a clear warning/message explaining that absolute paths are required, without actually failing

Actual behavior

"Error writing file" shown in red, but the file is written successfully. This is confusing — it looks like something went wrong when it didn't.

Environment

• Claude Code version: 2.1.63
• OS: macOS (Darwin 24.6.0)
• Shell: zsh