German and other non-ASCII keyboard input fails in the plan-editing Vim session

[aheritier]

Problem

German and other non-ASCII keyboard input does not work correctly in the Vim session used to edit a plan.

Context

Franck reported that German keyboard input, including non-ASCII characters, fails during plan editing in Vim.

Requested details

Please provide:

• docker-agent version/commit and installation method
• OS, locale, keyboard layout, and terminal emulator
• the exact command/configuration used to start docker-agent and open the plan editor
• Vim version and relevant Vim configuration
• which keys/characters fail, what appears instead, and whether ASCII input works
• the complete error text, if any, and, if possible, a screenshot or reproducible recording
• whether the issue reproduces outside sandbox mode and with another editor/terminal

Expected behavior

The Vim session used for plan editing should accept German and other non-ASCII keyboard input and preserve the entered characters correctly.

[aheritier]

Related issue: #3873 covers failure to launch external Vim when directly editing a plan in sandbox mode.