feat: shell completions (bash / zsh / fish / powershell)

[sauravpanda]

Problem

No shell completion support. Users have to remember every command, flag, and in some cases worktree numbers/branch names.

Suggested fix

Since bonsai uses cobra, this is essentially free:

// cmd/completion.go
var completionCmd = &cobra.Command{
    Use:   "completion [bash|zsh|fish|powershell]",
    ...
}

Bonus: add dynamic completion for commands that take a worktree number/branch (cd, rm, open, push) via ValidArgsFunction.

Add install instructions to README.