[Mzack9999]

Closes #774

Proposed changes

• The Host value used in log file names keeps only [A-Za-z0-9._:-]; every other character (path separators included) becomes _.
• The file logger refuses to write a file whose path is not directly inside the output folder.

Proof

New tests TestSaveRejectsNameOutsideFolder and TestStoreWriterKeepsFilesInLogFolder. pkg/logger and pkg/logger/file tests pass.