add OutgoingAttachment model, attachments field on OutgoingMessage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Packages/MagnumOpusCore/Sources/Models/OutgoingMessage.swift

@@ -1,3 +1,17 @@
+import Foundation
+
+public struct OutgoingAttachment: Sendable, Codable, Equatable {
+	public var filename: String
+	public var mimeType: String
+	public var data: Data
+
+	public init(filename: String, mimeType: String, data: Data) {
+		self.filename = filename
+		self.mimeType = mimeType
+		self.data = data
+	}
+}
+
 public struct OutgoingMessage: Sendable, Codable, Equatable {
 	public var from: EmailAddress
 	public var to: [EmailAddress]
@@ -8,6 +22,7 @@ public struct OutgoingMessage: Sendable, Codable, Equatable {
 	public var inReplyTo: String?
 	public var references: String?
 	public var messageId: String
+	public var attachments: [OutgoingAttachment]
 
 	public init(
 		from: EmailAddress,
@@ -18,7 +33,8 @@ public struct OutgoingMessage: Sendable, Codable, Equatable {
 		bodyText: String,
 		inReplyTo: String? = nil,
 		references: String? = nil,
-		messageId: String
+		messageId: String,
+		attachments: [OutgoingAttachment] = []
 	) {
 		self.from = from
 		self.to = to
@@ -29,5 +45,6 @@ public struct OutgoingMessage: Sendable, Codable, Equatable {
 		self.inReplyTo = inReplyTo
 		self.references = references
 		self.messageId = messageId
+		self.attachments = attachments
 	}
 }