Control-D should exit the application #3264

Rationale

• User Expectation: Ctrl-D is the standard UNIX/Linux convention for sending an EOF (end-of-file) signal, which typically exits interactive shells and REPL applications
• Muscle Memory: Users frequently press Ctrl-D out of habit when finishing with terminal applications (bash, zsh, python REPL, node REPL, psql, etc.)
• Consistency: Following this convention makes opencode more intuitive and reduces friction for developers familiar with terminal workflows
• Ergonomics: Ctrl-D is a quick, single-handed shortcut that is easier than typing exit commands or navigating menus

Current Behavior

Ctrl-D does not quit the application

Expected Behavior

Pressing Ctrl-D should cleanly exit the opencode TUI application, similar to how it works in shells and other interactive terminal programs

Implementation Notes

• May want to confirm before quitting if there are unsaved changes or active operations
• Should be documented in help/docs alongside other quit methods