fix(filesystem): add optional depth parameter to list_directory

[Will-hxw]

Summary

• Adds optional depth parameter to list_directory tool
• When depth > 1, recursively lists subdirectories with indentation
• Clamps depth to max 10 to prevent excessive recursion

Problem

LLMs (e.g., LM Studio with gpt-oss-120b-MLX-8bit model) are calling list_directory with a depth argument, but the schema rejected it with:

params is not allowed to have the additional property "depth"

Solution

1. Added depth: z.number().optional() to ListDirectoryArgsSchema
2. Updated inputSchema in tool registration
3. Implemented recursive listing with indentation for depth > 1

Test Plan

• npm run build passes
• Existing tests pass (2 pre-existing Unicode/root-dir failures unrelated to this change)