Publish command catalog, eliminate unknown types, and harden schema

[AndreyVMarkelov]

Summary

• Publish docs/json-schema/v1/commands.json as the public command contract catalog with a sync test against the internal golden fixture
• Remove unreachable unsupported_output_format error code from code and schema
• Add TestHelpOutputRemainsTextWithJSONFlag verifying help is always text regardless of --output
• Eliminate "unknown" metadata type: jsonMetadataFromDropbox now returns errors instead of silent fallbacks
• Skip metadata conversion in text mode for cp/mv (performance optimization)
• Remove "unknown" from all schema kind lists and add contract assertions rejecting it
• Validate error schema code enum matches all defined constants

Test plan

• All existing tests pass
• TestPublicJSONCommandCatalogMatchesGoldenContract prevents catalog/fixture drift
• TestHelpOutputRemainsTextWithJSONFlag covers root help, bare root, and command help
• TestJSONMetadataFromDropboxRejectsUnknownMetadata verifies nil metadata errors
• assertGoldenCommandStatuses rejects "unknown" in both statuses and kinds
• assertGoldenSuccessOutputStatuses validates all golden output results have explicit status/kind
• Error code enum validation matches all code constants
• gofmt, go vet, golangci-lint all pass