#11165: fix(cli): filter empty flags in zsh and PowerShell nested completion

## Summary - `generateZshArgs` uses `flags.split(/[ ,|]+/)` without filtering, so `flags[0]` (the fallback when no `--` flag exists) can be an empty string when `opt.flags` starts with a space or delimiter - `generatePowerShellCompletion` (nested commands at line 521) uses `flags.split(/[ ,|]+/)[0]` directly, same empty-string risk - Apply `.filter(Boolean)` after `.split()` to skip empty strings, consistent with the existing fixes in bash and PowerShell root completion ## Test plan - [x] Verified `.filter(Boolean)` removes empty strings from split result - [x] Consistent with existing bash completion fix and PowerShell root completion fix - [x] Pre-commit formatting passes Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates shell completion generation to avoid producing empty option tokens when an option’s `flags` string begins with whitespace or delimiters. In `src/cli/completion-cli.ts`, `generateZshArgs` now applies `.filter(Boolean)` after splitting `opt.flags`, and the PowerShell nested-completion path applies the same filtering to the computed option list. This aligns zsh + nested PowerShell behavior with the existing bash/root PowerShell completion logic that already avoids empty tokens, preventing empty-string completions and malformed completion entries. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is narrowly scoped to completion script generation and only removes empty tokens from flag-splitting; no behavior changes outside completion output, and it matches existing patterns already used elsewhere in this file. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->