Add a 'bpfcompat completion' command (bash/zsh/fish shell completion)

[ErenAri]

Summary

bpfcompat has no shell completion, so tab-completing subcommands and flags
isn't available. A bpfcompat completion <bash|zsh|fish> command is a small,
self-contained DX improvement and a friendly first contribution.

Task

• Add a completion subcommand that prints a completion script for
  bash/zsh/fish to stdout.
• Wire it in cmd/bpfcompat/ alongside the existing subcommand dispatch.
• Document it in the README install section
  (bpfcompat completion bash | sudo tee /etc/bash_completion.d/bpfcompat).

Acceptance

• bpfcompat completion bash emits a valid completion script (sourcing it
  enables subcommand/flag completion).
• A short test that the command runs and produces non-empty output per shell.

Self-contained and doesn't touch the VM/validation paths — good for getting
familiar with the CLI layer.