Add Duplicate File Finder mini-project

[zain-cs]

Summary

Adds a new mini-project: a CLI tool that finds duplicate files in a
directory based on content hashing (SHA-256), not just filename or size.

What it does

• Recursively scans a directory
• Groups files by size first (cheap pre-filter), then hashes remaining
  candidates to confirm true duplicates
• Reports duplicate groups and estimated reclaimable disk space
• Optional --delete flag to remove duplicates (with confirmation prompt)

Testing

Included a pytest suite covering:

• Hash consistency
• Detecting identical-content files
• Ignoring unique files
• Nested directory handling
• Empty directory edge case

All 5 tests pass locally. No new dependencies — standard library only.

Checklist

• Follows project style guidelines (single quotes, f-strings, 120-char lines)
• Tests added and passing
• README added
• No unrelated files changed

[github-actions]

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