Task: Merge AI Design image-to-image into current client main

Identity

Integrate the already reviewed AI Design single-reference image-to-image client change (809364e) onto the current client main baseline a03043d.
Preserve the already integrated local release-build and first-conversation latency work while resolving any overlapping README, page, test, or canonical documentation edits semantically.
Re-run the focused page suite, typecheck, scoped lint and Renderer build on the resulting integration head, then obtain an independent read-only review.
Land the verified result on local main only if its existing updater task can be left intact; never overwrite, stash, reset, or absorb that task's uncommitted files.

The picker continues to submit one real Workspace Asset ID through the existing attachmentAssetIds transport; no parallel upload or local-file description path is introduced.
Only brief.medium === 'image' selects image-reference semantics. video, null and missing medium retain the historical video-first-frame behavior.
Preserve Workspace / Conversation / Task ownership from ADR-001 and keep image/video tasks Workspace-scoped.
Preserve the deployed exact quick reply 从作品列表选择图片 for this compatible increment; a structured action/purpose remains future protocol work.
Keep all Git and file mutations rooted in this isolated integration worktree until the original main worktree is clean and ownership-safe.