[Feature] Drag and drop an image into the agent shell buffer

[dustinfarris]

Checklist

• I agree to communicate with the author myself (not AI-generated).
• I've read the README's Filing issues section.
• I'm running the latest versions (fill in below).
  • agent-shell version: 0.73.3
  • acp.el version: 0.13.1
  • ACP package (e.g. claude-agent-acp) version: claude-agent-acp 0.66.0
  • Agent CLI (e.g. claude, gemini) version: claude 2.1.233
• For requesting new agent support, I'm including a link to the ACP-capable agent or related ACP package.
• For issues, I'm including ACP traffic (as per README).

Request

Dragging an image file onto an agent-shell buffer should attach it to the prompt, the way agent-shell-send-clipboard-image and agent-shell-yank-dwim already do for the clipboard.

Current behaviour

agent-shell-mode doesn't handle drops, so a drop falls through to Emacs' default dnd-protocol-alist handling and the image is opened in its own buffer, taking over the window. Nothing reaches the prompt.

Suggested behaviour

On a file:// drop in an agent-shell buffer, insert the file as prompt context — the same thing agent-shell-yank-dwim already does once it has a path:

(agent-shell-insert
 :text (agent-shell--get-files-context :files (list path))
 :shell-buffer (agent-shell--shell-buffer))

A buffer-local dnd-protocol-alist entry set up in agent-shell-mode would cover it.

Two things worth deciding:

• Non-image files: attaching a dropped .log/.csv/source file as context seems just as useful, but falling back to dnd-open-file for anything non-image is a defensible scope too.
• Drops that carry image data rather than a file URL (dragging an image straight out of a browser) could reuse the existing save-to-.agent-shell/screenshots path from agent-shell--save-clipboard-image before inserting.

Why

Clipboard paste covers freshly-taken screenshots well. Drag-and-drop covers the other half: images that already exist as files — a design mockup, a diagram, a screenshot sitting in Finder or Dired — where today the route is copying the path out by hand.