Add Dropbox API error classification and contract audits

[AndreyVMarkelov]

Summary

• Classify Dropbox SDK errors into stable JSON error codes (auth_required, not_found, permission_denied, rate_limited, dropbox_api_error, path_conflict) via typed error matching, reflection-based summary extraction, and message parsing
• Add coded error factories for auth flows (auth_required, app_key_required, auth_exchange_failed, auth_refresh_failed) and wire them into all auth error paths
• Introduce golden contract tests: schema audit validates definitions and per-command schemas against a fixture, success output audit validates representative example payloads, and fixture coverage audit ensures every structured command has test coverage
• Document all JSON error codes in README table

Test plan

• All existing tests pass
• New TestJSONErrorCodeClassifiesDropboxAPIErrors covers typed SDK errors, wrapped errors, and string-format errors
• TestJSONErrorCodeDoesNotClassifyPlainValidationStrings prevents false positives
• Auth tests assert correct error codes on all failure paths
• TestStructuredOutputGoldenSchemaAudit validates schema fixture against code-derived schemas
• TestStructuredOutputGoldenSuccessOutputAudit validates golden JSON examples match code
• TestStructuredOutputSuccessFixtureAudit verifies test function references exist
• gofmt, go vet, golangci-lint all pass