Return the shell command that adds a package to the dcode tool env.
The result is built for execution (via perform_install_package), not for
display — surfacing raw uv tool invocations to the user is intentionally
avoided. package is validated and then shlex.quote-d: the validation
already blocks shell metacharacters, so the quoting is defense in depth that
keeps the command safe even if the pattern is later loosened.
Already-installed extras are folded into the deepagents-code[...]
requirement via the shared _dcode_extras_requirement helper, the same way
install_extras_command builds its requirement. Without this the reinstall
would replace the tool with a plain deepagents-code, silently dropping any
extras the user added through /install <extra>.
