feat: add shell completion generation #132

Problem

ct has a growing command tree with nested commands and options, but currently offers no way to generate shell completion. Users must remember or repeatedly inspect commands such as ct report permissions --help.

Proposed CLI

Add a top-level command that writes a completion script to stdout:

ct completion zsh
ct completion bash
ct completion fish

Example installation for zsh:

mkdir -p ~/.zfunc
ct completion zsh > ~/.zfunc/_ct

The user's ~/.zshrc can then load it through fpath and compinit.

Requirements

• Generate completion from the Commander command structure where practical, so commands and options do not drift from the actual CLI.
• Complete top-level commands, nested subcommands, options, and enumerated option values.
• Include current nested surfaces such as report permissions and its --by-subject, --by-object, and --by-both options.
• Use filename/path completion for options that accept files or directories.
• Work from both the npm installation and standalone binaries.
• Document installation for zsh, bash, and fish.
• Add tests that verify representative nested commands and options are present in each generated script.
• Writing a completion script must not contact ChurchTools or require authentication.

Acceptance criteria

ct completion zsh  > /tmp/_ct
ct completion bash > /tmp/ct.bash
ct completion fish > /tmp/ct.fish

Each command exits successfully without credentials, emits a non-empty script, and the installed completion offers nested commands/options matching ct --help and the relevant subcommand help output.