test(ls): complete unit test coverage (#136)

* test(ls): complete unit test coverage

Add 43 new tests covering all ls command functions:
- filterAndSortFiles edge cases and sorting
- buildChunkIndex with fallback logic
- displayLongFormat with all flag combinations
- FormatSharedWith truncation scenarios
- DisplayShortFormat output formatting

Preserved 5 existing tests. Total 48 tests passing.

* fix: format Go files (gofmt)

Author: Janmesh23