Support capability-gated clipboard image paste over ACP

[jbellis]

Goal

Allow pasting an image from the system clipboard into the Hel chat composer when the active ACP agent advertises prompt_capabilities.image.

Scope

• Detect clipboard image data on Ctrl-V or Cmd-V after preserving normal text-paste behavior.
• Enable the action only when the ACP agent advertises image prompt support.
• Encode and send an ACP ImageContent block with base64 data and the correct MIME type.
• Preserve Alt-V voice dictation.
• Cover macOS, Windows, X11, and Wayland clipboard behavior, and document terminal or tmux limitations.
• Keep image data in memory where practical; if temporary files are necessary, clean them up on cancellation and errors.
• Add behavior tests for supported and unsupported agents, text-versus-image precedence, clipboard failures, and cleanup.

Context

Text clipboard paste is being added with arboard. Image support is intentionally deferred from that change to avoid scope creep.