Normalize multi-result commands to shared JSON output types

[AndreyVMarkelov]

Summary

• Consolidates cp, mv, put, get, and rm to use shared jsonOperationOutput with consistent input/results/warnings structure
• All multi-result JSON responses now include top-level "input", "results", and "warnings": [] fields
• Removes per-command output wrapper structs (relocationOutput, putOutputData, getOutputData, removeOutput) in favor of shared renderJSONOperationOutput helper
• Adds newJSONOperationResult constructor and per-command converter functions
• Updates README JSON examples to reflect the normalized schema

Test plan

• All existing tests pass
• Test decode helpers now assert warnings is always [] (not null)
• New tests for renderJSONOperationOutput and newJSONOperationResult
• gofmt, go vet, golangci-lint clean