Add `eko size` command to report snapshot disk usage

[kavix]

🎯 Goal

Implement an eko size command that walks .eko/snapshots/ and reports disk usage per snapshot (in KB/MB/GB) as well as a grand total, so developers can understand how much storage their history is consuming.

💡 Expected Output

$ eko size
ID         Created              Size
8c9d1a2f   2026-08-08 20:11    42.3 MB
3b7f2a1e   2026-08-07 14:32    40.1 MB
1f4a9b0c   2026-08-06 10:05    39.8 MB
──────────────────────────────────────
Total: 3 snapshots · 122.2 MB

🛠️ Implementation Steps

1. Create cmd/size.go with a Cobra command sizeCmd.
2. Walk .eko/snapshots/<id>/ using filepath.WalkDir and sum file sizes.
3. Format sizes with human-readable units (KB, MB, GB).
4. Support --json flag for machine-readable output.
5. Add unit tests in cmd/commands_test.go.

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📖 See full details in GOOD_FIRST_ISSUES.md — Issue #14
🌟 Great for first-time contributors! Read CONTRIBUTING.md to get started.