Add --output=json support to put command

[AndreyVMarkelov]

Summary

• Refactors put command to support structured JSON output via --output=json
• Upload functions (uploadSingleShot, uploadChunked) now return *files.FileMetadata for result reporting
• Adds putResult with status tracking (uploaded, skipped, created, existing) and kind (file, folder)
• Recursive directory handling reports per-file/folder results in a single JSON array
• Stdin uploads hide internal temp spool paths from JSON output
• Adds nil checks in jsonMetadataFromDropbox for typed nil interfaces
• Updates README with put JSON example and migrated command list

Test plan

• All existing tests pass
• New JSON tests: single file upload, default target, if-exists skip/fail, stdin (verifies temp path not leaked), recursive with directories and files, text mode silent
• gofmt, go vet, golangci-lint clean