fix: normalize object store prefixes

[rmanibus]

Closes #147

Summary

• normalize configured object-store prefixes so repository objects stay under the intended prefix even when the store URI omits a trailing slash
• apply the normalization to both S3 and B2 prefix options
• add regression tests for prefix normalization and prefixed key behavior

Testing

• go test ./pkg/store ./cmd/cloudstic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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