Fix dotnet graph-api-skills sample

[MGudgin]

The skillParentDirectory local used to find the SummarizeSkill in the graph-api-skills sample had one too few levels of directory navigation, resulting in a DirectoryNotFoundException.

Initialize variable using RepoFiles.SampleSkillsPath from kernel-syntax-examples project.

Reference kernel-syntax-examples project.

Make RepoFiles class and SampleSkillsPath method public in kernel-syntax-examples.

Add namespace to graph-api-skills/Program.cs to avoid type collision with Program class in kernel-syntax-examples project.