Show real-time progress bar during `eko save` and `eko restore`

[kavix]

🎯 Goal

For large projects with thousands of files, eko save and eko restore give no visual feedback. Add an optional real-time progress bar (file count / total) printed to stderr so users can see the operation progressing.

💡 Expected Behavior

$ eko save
Saving snapshot... [████████████████░░░░] 1234/1500 files
Snapshot saved: 8c9d1a2f

🛠️ Implementation Steps

1. Add a --progress flag (default true when stdout is a TTY) to cmd/save.go and cmd/restore.go.
2. Use an atomic counter in internal/util/fs.go worker pool to track copied file count.
3. Print a carriage-return-based progress line to stderr using ANSI escape codes (no external dependency needed).
4. Add unit tests verifying the counter increments correctly.

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📖 See full details in GOOD_FIRST_ISSUES.md — Issue #13
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