fix(ci): enable shell completion verification

[albert20260301]

Summary

• install Fish and Zsh before canonical verification so the executable completion-contract tests added in feat(cli): simplify the command API surface area #75 run in CI instead of being skipped

Validation

• bun test src/commands/completion/lib/spec.test.ts — 57 passing (Bash, Fish, and Zsh)
• bun run lint

[albert20260301]

@francoischalifour could you review this?

I picked you because you authored the completion-contract refactor in #75. CI is green. The main review focus is whether canonical verification should install Fish and Zsh so those executable shell contracts run instead of skip.