Add safe directory operations to plugin filesystems

[matteomekhail]

Summary

• add backward-compatible directory creation/deletion methods to scoped filesystems
• implement them for persistent plugin files, cache files, and execution sandboxes
• require explicit recursive deletion and forbid deleting sandbox roots
• add regression tests and plugin-author documentation

Validation

• ./gradlew test
• ./gradlew :composeApp:jvmTest --tests org.wip.plugintoolkit.features.plugin.logic.DefaultExecutionFileSystemTest

Closes #10