refactor: simplify Target and Remote layering - #2333

What this PR does / why we need it:

Simplifies the Target/Remote layering in cmd/oras/internal/option:

• Short-circuits Target.ModifyError on target.Type == TargetTypeOCILayout || target.IsOCILayout so --oci-layout-path targets do not fall through into remote registry error formatting.
• Removes the duplicate Target.prefix field and uses Remote.flagPrefix consistently across setFlagDetails, ApplyFlags, and Parse.
• Shares the TargetTypeOCILayout / TargetTypeRemote dispatch across NewTarget, NewBlobDeleter, NewManifestDeleter, and NewReadonlyTarget.
• Initializes remo.store on demand via credentialStore() so Remote.ConfigPath() and DecorateCredentialError() do not depend on authClient having run first.

Which issue(s) this PR fixes:

Relates to #2280

Please check the following list:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline comments, but also with conceptual docs such as an overview of a new feature, or task-based documentation like a tutorial?