feat: make gathered log directory configurable

[bdegeeter]

Summary

Allow callers to configure where gathered machine logs are written.

matcher.LogDir remains a public package variable and defaults to logs, preserving the existing behavior for callers that do not configure it.

Changes

• Add the public matcher.LogDir configuration variable.
• Support absolute and nested log directories.
• Create missing parent directories with os.MkdirAll.
• Construct destination paths with filepath.Join.
• Preserve the basename of the remote log file.
• Report directory and file creation failures without dereferencing a nil file.
• Avoid closing the SCP client more than once.

Motivation

CI environments commonly require logs to be written directly into a designated artifact directory. Previously, gathered logs were always written to a hard-coded logs directory.

Compatibility

The default remains logs, so existing users retain the current behavior unless they explicitly change matcher.LogDir.

Validation

The following checks pass with Go 1.24.2:

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check

Unit tests cover:

• The default directory.
• Absolute and nested directories.
• Preserved filenames.
• Invalid directory paths.
• Invalid file destinations.