Add JSON schema envelope and normalize all output fields

[AndreyVMarkelov]

Summary

• Add ok, schema_version, and command fields to every JSON response (success and error)
• Remove omitempty from operation result fields — status/kind default to "unknown", input/result default to {}
• Add explicit status constants for account (found), du (reported), version (reported), rm (deleted/permanently_deleted), cp (copied), mv (moved)
• Publish formal JSON Schema files under docs/json-schema/v1/ with contract tests validating them
• Validate error schema code enum matches all defined error code constants
• Assert every command schema has at least one non-unknown status

Test plan

• All existing tests pass
• TestPublicJSONSchemaFiles validates schema files match code (required fields, properties, const values, error code enum)
• TestStructuredOutputGoldenSchemaAudit validates updated schemas with new envelope fields and explicit statuses
• TestStructuredOutputGoldenSuccessOutputAudit validates updated golden examples with envelope
• TestRenderCommandErrorWritesJSONErrorToStdout verifies schema_version and command on errors
• TestRenderJSONOperationOutput verifies ok/schema_version/command on success
• TestNewJSONOperationResultNormalizesEmptyFields verifies unknown defaults
• assertGoldenCommandStatuses ensures no command has empty or "unknown" status list
• gofmt, go vet, golangci-lint all pass