Add --depth flag to limit directory traversal depth

[MatthewMckee4]

Summary

Add a --depth <N> flag to limit how deep kount recurses into directories. Useful for getting a high-level overview of a project without descending into deeply nested dependencies or vendored code.

Example

# Only count files in the immediate directory
kount --depth 1 .

# Count up to 3 levels deep
kount --depth 3 src/

Acceptance criteria

• --depth <N> limits directory traversal to N levels
• --depth 0 counts only explicitly provided files
• --depth 1 counts files in the given directory but not subdirectories
• Works with the ignore crate's max_depth setting