feat(completion): add nushell and carapace completion generators

[schoolboyqueue]

Summary

Add custom completion generators for shells not natively supported by Cobra:

• Nushell: Generates native extern definitions that integrate with nushell's completion system
• Carapace: Generates YAML spec file for use with carapace-bin, enabling completions for Elvish, Ion, Oil, Tcsh, Xonsh, and other shells

Changes

• Add internal/cli/admin/completion_generators.go with custom generators
• Register new nushell and carapace subcommands under completion
• Add comprehensive tests for both generators
• Update docs/public/SHELL-COMPLETION.md with setup instructions

Test plan

• All existing completion tests pass
• New tests verify nushell extern format output
• New tests verify carapace YAML spec output
• make lint passes
• make test passes
• make build passes

Usage

# Generate Nushell completions
autospec completion nushell > ~/.cache/autospec/completions.nu

# Generate Carapace spec
autospec completion carapace > ~/.config/carapace/specs/autospec.yaml