fix: use detected media type for screenshots instead of hard-coded image/png

browser_screenshot declared every capture as image/png regardless of the
actual bytes. On Chromium/Edge builds where captureVisibleTab returns JPEG
bytes, the stored attachment reference recorded mediaType "image/png" against
JPEG bytes, so dsh-attachment-local's read-time metadata verification fails
with ATTACHMENT_CORRUPT ("Stored attachment metadata does not match its
reference"), surfacing as code UNKNOWN and triggering endless LLM retries.

- image.ts: sniff the capture's magic bytes (PNG/JPEG/WebP/GIF) and declare
  that real media type to attachments.saveImage, instead of always "image/png".
- tools.ts: assemble the tool result from ref.mediaType (the store's verified
  type) rather than re-declaring "image/png" as const, and relax the output
  schema's const to the accepted media-type enum.
- tests: add a regression case asserting JPEG capture bytes surface as
  image/jpeg.

Co-Authored-By: DeepSeek Harness <noreply@deepseek.com>

packages/dsh-plugin-browserskill/src/tools.ts

@@ -772,7 +772,11 @@ export function registerTools(deps: ToolDeps): () => void {
               additionalProperties: false,
               properties: {
                 attachmentId: { type: "string", required: true },
-                mediaType: { type: "string", required: true, const: "image/png" },
+                mediaType: {
+                  type: "string",
+                  required: true,
+                  enum: ["image/png", "image/jpeg", "image/webp", "image/gif"],
+                },
                 bytes: { type: "integer", required: true },
                 width: { type: "integer", required: true },
                 height: { type: "integer", required: true },
@@ -842,7 +846,7 @@ export function registerTools(deps: ToolDeps): () => void {
               ? {
                   image: {
                     attachmentId: String(ref.attachmentId),
-                    mediaType: "image/png" as const,
+                    mediaType: ref.mediaType,
                     bytes: ref.bytes,
                     width: ref.width,
                     height: ref.height,