bash_bg tool row shows the plain command line

The bash_bg tool row (the TUI entry for the call in an agent turn) showed only the bold tool name, with the command visible nowhere in the row. The row now renders the command as $ <command>, the same line shape as the built-in bash row, with no background marker. The tool name "Bash background" already says the command is backgrounded, and the command line is the true intent.

Considered options: a $ <command> & suffix, faithful to the shell but repeating what the tool name already says; a bg $ <command> prefix, which breaks the $ line shape the operator scans across the bash rows beside it. Both were rejected in favor of the plain line.