Fix filename handling bug in Sietch add operation

[MrKeiKun]

Summary

The Sietch add operation was incorrectly using filepath.Base(pair.Source) for manifest storage, causing files with identical names from different directories to overwrite each other's manifests.

AI Assistance

• I used AI assistance (generate code, write tests, refactor, or docs)
  • If yes, briefly describe where and how:

Scope & Risk

• Adds tests or updates existing tests
• Security/privacy reviewed where applicable
• Performance/complexity considered

Hacktoberfest Notes

• Link the issue this PR resolves (required for consideration): add operation not storing the name of the file correctly. #138
• Explain the problem and your approach in your own words (1-3 sentences):
  There was a typo on add.go line 257. It should have used destFileName rather filepath.Base(pair.Source) making files with identical names from different directories were overwriting each other's manifests. I also added a test there just to be safe.

Checklist

• PR title is descriptive
• Commits are meaningful (no bulk AI dumps)
• No generated files or lockfile-only changes unless justified

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/add.go	0.00%	1 Missing ⚠️

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