Add empty-input guard in parseRepositoryImportJSON

[morluto]

Problem

parseRepositoryImportJSON directly reads data[0] without an empty-input guard:

if data[0] == '[' {

The function is currently only called from parseRepositoryImport after trimming and a non-empty check, so this has not been user-visible. But as a shared parser helper it can panic if called with empty data in future code paths (or refactors), and tests could regress this by accidentally passing an empty slice.

Evidence

• File: internal/cli/cli.go line around 1001
• Direct index in parseRepositoryImportJSON without len(data) == 0 check.
• Called only by parseRepositoryImport today, but not protected as a standalone parser contract.

Impact

Potential index out of range panic and crash if the helper is reused without replicating the current precondition in every caller.

Suggested fix

Add a small defensive guard:

if len(data) == 0 {
    return nil, errors.New("repository import payload is empty")
}

[morluto]

Fixed by the merged boundary-parser work in #247. Empty repository-import payloads are rejected as errors rather than reaching an unchecked index.