fix(filesystem): reject move_file when destination exists

[re2zero]

Closes #4628

Problem

move_file silently overwrites an existing destination file even though the README and the tool description both state it will fail if the destination exists. This is a data-loss bug: an agent that trusts the documented safety contract can destroy files.

Fix

Add an existence check via fs.stat before fs.rename. If the destination already exists, an error is thrown immediately, matching the documented contract.

try {
  await fs.stat(validDestPath);
  throw new Error(`Destination already exists: ${args.destination}`);
} catch (err: any) {
  if (err instanceof Error && err.message.startsWith("Destination already exists")) {
    throw err;
  }
  if ((err as NodeJS.ErrnoException).code !== "ENOENT") {
    throw err;
  }
}
await fs.rename(validSourcePath, validDestPath);

Testing

• server-filesystem test suite: 152 passed (7 test files).