#17437: fix(completion): avoid rc=1 for optionless zsh leaf commands

## Summary - fix zsh completion generation for leaf commands that have no options - avoid emitting an empty `_arguments -C` block for those leaves - emit a no-op leaf function (`return 0`) instead - add a regression test in `src/wizard/onboarding.completion.test.ts` ## Why Some generated zsh leaf functions ended up with an empty `_arguments -C` body, which returned non-zero (rc=1) when invoked. This makes completion behavior brittle for optionless leaf commands. ## Verification - `npx pnpm vitest src/wizard/onboarding.completion.test.ts` ✅ - `npx pnpm exec oxfmt --check src/cli/completion-cli.ts src/wizard/onboarding.completion.test.ts` ✅ - `npx pnpm exec oxlint --type-aware src/cli/completion-cli.ts src/wizard/onboarding.completion.test.ts` ✅ - manual repro with generated mini zsh completion: `_openclaw_config_unset` returns `rc:0` ✅ <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixed zsh completion generation for leaf commands without options by emitting a no-op function (`return 0`) instead of an empty `_arguments -C` block that would fail with rc=1. - Refactored `generateZshSubcommands` to handle three cases: commands with subcommands, leaf commands with options, and leaf commands without options - Added regression test to verify no-op function generation for optionless commands - Optimized argument line generation to avoid trailing backslash when no args exist <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The fix is surgical and well-targeted: it addresses a specific bug in zsh completion generation for optionless leaf commands, includes a regression test, and follows the existing code patterns. The logic flow is correct across all three cases (commands with subcommands, leaf with options, leaf without options). - No files require special attention _{Last reviewed commit: e949bf7} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->