fix: preserve image context across follow-up requests

Image requests previously sent only the current prompt and attachments, so
follow-up edits could lose the previous image and conversation requirements.
Reuse the latest successful image from the existing artifact store together
with text history, including after reopening the app. Explicit attachments
take priority, and configured model IDs remain unchanged.

When image editing is explicitly unsupported, continue with text-based image
generation and retain a quiet message footer explaining the missing image
context. Keep genuine provider errors and cancellation visible. Add focused
regressions, persisted footer metadata, and aligned feature documentation.

Validated with four real sub-gpt-image-2 image requests and a simulated
unsupported-edit response. Image regressions, typecheck and lint passed;
the full working-tree run had two unrelated concurrent Runtime-menu failures.

Release note: 连续改图会自动带上上一张图片和文字上下文，重开对话后仍可继续修改。上游明确不支持编辑时仍可按文字生图，仅在回复底部轻提示未使用参考图片。

FEATURES.md

@@ -104,6 +104,11 @@ otherwise.
   editing through providers supporting the OpenAI-compatible image-editing
   endpoint. Editing sends attached images as multipart data and still requires
   validated inline image output rather than fetching provider-returned URLs.
+  Follow-up image requests reuse the latest successful image and conversation
+  text, including after reopening a saved conversation; explicit attachments
+  take priority. Missing editing support does not block text-based generation:
+  the affected reply shows a quiet footer explaining that reference images were
+  not used. See [image generation](./docs/features/image-generation/README.md).
 - [x] **Direct model programming agent**: Local direct text models can run the
   platform Shell in Execute mode and delegate one level of programming
   Subagent work while inheriting the parent request's mode, model, workspace,