fix: emit valid zsh completions for nested groups

[hlship]

Summary

• Nested group completions now use the next path segment instead of a multi-word command path, so generated zsh scripts parse (zsh -n).
• #compdef binds the real tool name, and -V/--version is included when dispatch has :version.

Fixes #72

Test plan

• clj -M:test -m cognitect.test-runner -n net.lewisship.cli-tools.completions-test
• Generate completions for a tool with nested groups and run zsh -n on the file
• Confirm compinit defines nested functions such as _tool_group and _tool_group_nested
• Confirm -V/--version appears when the tool sets :version