Add `eko diff <id1> <id2>` terminal file diff command

[kavix]

🎯 Goal

Add an eko diff subcommand that displays line-by-line unified colored diffs between two snapshot points directly in the terminal interface.

💡 Expected Usage

# Compare latest snapshot with predecessor
eko diff

# Compare specific snapshot with current workspace
eko diff 8c9d1a2f

# Compare two specific snapshots
eko diff 3b7f2a1e 8c9d1a2f

🛠️ Implementation Steps

1. Create cmd/diff.go with Cobra command diffCmd.
2. Integrate a lightweight Go diff algorithm (e.g. sergi/go-diff or stdlib text comparison).
3. Print unified git-style + / - colored line diffs.
4. Add unit tests in cmd/commands_test.go.

---

📖 See full details in GOOD_FIRST_ISSUES.md — Issue #6
🌟 Great for first-time contributors! Read CONTRIBUTING.md to get started.