Fix unknown command suggestions

[Wenjunyun123]

Summary:

• Adds a maximum edit-distance threshold before suggesting a command for unknown input.
• Keeps close typo suggestions while avoiding unrelated suggestions.
• Adds regression coverage for close and distant unknown commands.

Testing:

• git diff --check
• Not run: go test ./pkg/cmd -run TestSuggestCommand because go is not installed in this local environment.

Fixes #16

AI assistance: This bug fix was prepared with AI assistance and reviewed before submission.