feat: add directory-level disk usage stats

[blinkagent]

Summary

Currently, clistat only supports disk usage at the block device/filesystem level via syscall.Statfs. This reports total filesystem usage, which isn't ideal in Docker containers where multiple directories share the same underlying filesystem but you want to track usage of specific directories (e.g., /home or /var/lib/docker).

Problem

In containerized environments:

• The current Disk() function shows the entire host filesystem usage
• Users need visibility into actual disk usage of specific directories within the container
• This is particularly important for Docker-in-Docker setups where /home and /var/lib/docker need separate tracking

Proposed Solution

Add a new function (e.g., DiskUsage or DirectoryUsage) that calculates the actual disk usage of a directory tree, similar to du -sh.

Considerations:

• Performance: du-style operations can be expensive for large directory trees with many small files (e.g., node_modules). May need to consider:
  • Caching strategies
  • Configurable depth limits
  • Recommended refresh intervals (current workaround uses 1h intervals)
• Should complement, not replace, the existing Disk() function which is still useful for filesystem-level stats

Use Case

This would enable accurate disk usage monitoring in agent metadata for containerized workspaces, showing real directory usage instead of host filesystem totals.

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Created on behalf of @matifali