Add shell completions (bash, zsh, fish)

## Description

Generate and distribute shell completions via clap.

## Tasks

- [ ] Enable clap shell completion generation (bash, zsh, fish)
- [ ] Include completions in binary releases or document `lineark completions <shell>` command

## Blockers

- #15 — CLI commands and help text must be finalized

## References

- clap has built-in completion generation support