Return RuntimeError when --deduplicate-hardlinks is used with multiple arguments

[Copilot]

Using --deduplicate-hardlinks with multiple path arguments is inherently unreliable across device boundaries and overlapping trees. Rather than attempting to silently fix up the argument list, pdu now rejects this combination outright.

Changes

• New RuntimeError variant: DeduplicateHardlinkMultipleArgs (exit code 7, #[cfg(unix)]) — emitted when --deduplicate-hardlinks is passed alongside more than one path argument
• Removed overlapping_arguments module: the path-deduplication logic (remove_overlapping_paths, find_overlapping_paths_to_remove, etc.) is no longer needed and has been deleted entirely
• App::run() no longer takes mut self: previously required to mutate args.files in-place
• Updated tests: hardlinks_deduplication_multi_args.rs now asserts the error path instead of the old overlapping-removal behavior

$ pdu --deduplicate-hardlinks dir1 dir2
[error] DeduplicateHardlinkMultipleArgs: --deduplicate-hardlinks cannot be used with multiple arguments

Closes #347

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[github-actions]

Performance Regression Reports

commit: 6d86325

There are no regressions.