Make prompt images deletable (without selection)

[cxa]

Problem:

• In agent-shell-mode, when the prompt contains an inline image, C-d with point on the image and DEL with point just after the image do not remove it.

Fix:

• Add explicit agent-shell-mode bindings for C-d, DEL, <backspace>, and <delete> so the expected delete keys are available in the mode.
• Route image deletion through delete-field.
• Mark only inline image attachments with a private field symbol: agent-shell--image-field.
• Leave non-image file attachments unchanged.

Tests:

• Added a test that inline image context is marked with the image field.
• Added a test that non-image file context does not get that field.
• Added a forward-delete test for deleting an inline image.
• Added a backward-delete test for deleting an inline image.
• Added a keybinding test for the delete keys in agent-shell-mode.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've added tests where applicable.

[xenodium]

Thank you for the PR!

I've submitted 92f99ad to make deletion more generic. Should work with any other binding users are used to and also cover both the shell and compose buffers. Can you give that a try and see if it works for your use case?

[cxa]

Works, thanks!!!