fix(cli): reject unknown completion shells

[upsetbit]

Summary

Fixes #298.

Rejects unknown shells passed to prosa completion instead of printing completion help with exit 0.

What changed

• Initializes Cobra's default completion command during root command construction.
• Adds validation for the supported shells: bash, zsh, fish, and powershell.
• Keeps prosa completion without a shell as a help-only command with exit 0.

Validation

• go test ./internal/cli -run 'TestCompletion|TestParentCommandsRequireExplicitSubcommands' -count=1
• go test ./internal/cli/... -race
• go build -o ./bin/prosa ./cmd/prosa
• Manual checks for prosa completion wat, prosa completion, and prosa completion bash