Problem

There's no way to download file attachments from the CLI. After #119 added files metadata to message JSON output, consumers can see that files exist but can't retrieve them without manually curling with auth headers.

Expected behavior

slck files download <file-id-or-url> [--output <path>] should download a Slack file to the local filesystem.

Scope

• New files subcommand group with a download command
• Support file ID or url_private as input
• Default output to current directory using the file's original name
• --output flag to specify a custom path