carry attached images through to the model

The composer already uploaded an image, split its data URL and put the bytes on the wire as
`images`. Nothing on the server read them. The `chat` ClientMessage had no such field, and the
prompt reached the sidecar as a bare string, so all the model ever saw was the client-generated
`[Attached image: …]` placeholder — a label describing a picture it was never shown.

The transport was never the obstacle: `query()` consumes an async iterable of user messages whose
`content` is an Anthropic `MessageParam`, and only `pushTurn` hardcoding a string kept it to text.
So `images` is threaded through the four hops that dropped it and turned into native image content
blocks at the end, renaming `mediaType` to the API's `media_type` at that last step.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/sidecar/protocol.ts

@@ -1,4 +1,4 @@
-import type { MessageCost, TurnMessage } from '../api/chat/types';
+import type { MessageCost, PromptImage, TurnMessage } from '../api/chat/types';
 
 // ── Envelope ──
 
@@ -121,6 +121,7 @@ export type ClaudeSpawnStreamingParams = {
   email: string;
   username: string;
   prompt: string;
+  images?: PromptImage[]; // sent as native image content blocks alongside the prompt text
   sessionKey: string;
   cwd?: string;
   model?: string;