feat(chat): #155 paste image depuis presse-papier dans le composer custom (QA verte)

- frontend: onPaste sur CustomAgentChatView, détection MIME image, chip/preview d'attachment, envoi via le contrat #154 (adapters/agent, ports)

- backend: import d'image par bytes dans le store attachments (commands, chat_attachments app+infra, dto, ports) + tests

frontend/src/ports/index.ts

@@ -145,6 +145,25 @@ export interface CreateAgentInput {
   initialContent?: string;
 }
 
+export type ChatAttachmentSourceKind =
+  | "localFile"
+  | "clipboard"
+  | "dragDrop"
+  | "other";
+
+/** Structured attachment intent accepted by `agent_send`. */
+export interface ChatAttachmentInput {
+  path?: string;
+  filename?: string;
+  contentBase64?: string;
+  mime?: string;
+  sourceKind?: ChatAttachmentSourceKind;
+}
+
+export interface SendAgentChatOptions {
+  attachments?: ChatAttachmentInput[];
+}
+
 /**
  * Best-effort enriched details about a CLI conversation (T7), used to enrich the
  * resume popup. Both fields are optional: a missing inspector or a missing
@@ -284,6 +303,7 @@ export interface AgentGateway {
     sessionId: string,
     prompt: string,
     onChunk: (chunk: ReplyChunk) => void,
+    options?: SendAgentChatOptions,
   ): Promise<void>;
   /** Interrupts only the current turn of a live structured session. */
   cancelAgentChat?(sessionId: string): Promise<void>;