#6382: fix(cli): silence Powerlevel10k Zsh warning from completion script

Fix issue #6357 Suppress stderr when using openclaw completion. The stderr output: ``` (node:291720) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead. (Use `node --trace-deprecation ...` to show where the warning was created) ``` <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the zsh installation snippet written into `~/.zshrc` to redirect stderr for the `openclaw completion --shell zsh` process substitution, suppressing Node’s `DEP0040` deprecation warning during shell startup/completion loading. The change is localized to `src/cli/completion-cli.ts` where the install command assembles the `source <(...)` line for zsh profiles. <h3>Confidence Score: 4/5</h3> - This PR is low-risk and likely safe to merge, with the main concern being reduced debuggability due to blanket stderr suppression. - The change is a one-line modification to the zsh completion install command; it doesn’t affect runtime CLI behavior outside of the installed shell snippet. The primary trade-off is that redirecting all stderr may hide legitimate errors when generating completion. - src/cli/completion-cli.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->