refactor(Mountain): Decompose FileSystem module and add native readdir handler

Replace the FileSystem glob import with explicit submodule paths for all Managed (trait-based) and Native (tokio::fs) file operations. Add new FileReaddirNative handler under FileSystem/Native/ implementing the file:readdir wire method using tokio::fs.

This continues the module decomposition pattern from recent commits, replacing the monolithic FileSystem.rs with granular domain-specific handlers. The explicit paths follow CLAUDE.md requirements, making dependency relationships clear at every call site.

No functional changes - this is a pure refactoring for code organization and maintainability.

Author: NikolaRHristov