docs: add shell completion examples

[GHX5T-SOL]

Closes #1.

Summary:

• add a bash completion example for the common OpenSOP subcommands
• add a zsh completion file example with fpath/compinit install notes
• keep this docs-only because the CLI does not generate completions yet

Validation:

• bash -n bin/opensop passed
• sed -n '/_opensop_complete()/,/complete -F/p' README.md | bash -n passed
• awk '/^```zsh$/{flag=1; next} /^```$/{if(flag){flag=0; next}} flag{print}' README.md | zsh -n passed
• git diff --check passed
• git diff --no-ext-diff | gitleaks stdin --no-banner --redact --exit-code 1 passed, no leaks found