feat(terminal): word and line editing keys in the embedded terminal

Option+Left/Right/Backspace/Delete now send readline Meta sequences and
Cmd+Left/Right/Backspace/Delete send ^A/^E/^U/^K, matching what native
macOS terminals emit. Default TERM to xterm-256color when the app was
launched without one, so zle/readline key handling works from Finder.

Co-Authored-By: Claude <noreply@anthropic.com>