Image dimension limit (2000px) fails silently after submit, forcing /compact and context loss

Summary

When attaching multiple images, if any single image exceeds the 2000px dimension limit, the whole turn fails with:

An image in the conversation exceeds the dimension limit for many-image requests (2000px). Run /compact to remove old images from context, or start a new session.

The error only appears after the user submits — and the only suggested fix (/compact) destroys conversation context the user may want to keep.

Impact

• User has no way to know which image is oversized before submitting.
• User loses context (via /compact) for a problem that was fundamentally a client-side validation miss.
• Esc Esc (Rewind) doesn't help because the failed turn never created a checkpoint.
• Extremely frustrating when attaching batches of screenshots (common workflow).

Requested improvements

1. Validate dimensions at attach time, not at send time. Show a warning the moment the image is dragged/pasted.
2. Identify which image is oversized in the error message (name/index/dimensions).
3. Offer auto-resize on attach (e.g. "This image is 3200x2400, resize to 1800px?") instead of silently accepting and failing later.
4. Stop recommending /compact as the only fix — it's destructive and unrelated to the root cause. At minimum suggest removing just the offending image.
5. Create a rewind checkpoint even for failed turns, so users can Esc Esc back to just before the bad attach.

Reproduction

1. Attach several images to a message, with at least one >2000px on either axis.
2. Submit.
3. Turn fails with the error above. No indication of which image is the problem. /compact is the only suggested recovery.

Environment

• Claude Code CLI
• macOS
• Workflow: attaching batches of screenshots for analysis