feat(files): add files download command

[piekstra]

Summary

• Add slck files download <file-id-or-url> command to download Slack file attachments
• Accepts bare file ID (F0AHF3NUSQK) or any Slack file URL (url_private, url_private_download, permalink)
• --output / -O flag to specify destination path; defaults to original filename in cwd
• New files subcommand group (extensible for future file commands)
• New client methods: GetFileInfo (files.info API) and DownloadFile (authenticated download)

Why

After #119 added file metadata to message JSON output, consumers could see that files existed but had no CLI way to retrieve them — downloading required manually curling with auth headers. This closes the loop so you can go from messages thread --output json to files download <id> in a single workflow.

Test plan

• Unit tests for file ID resolution (bare ID, url_private, url_private_download, permalink, invalid input)
• Unit tests for download (success, URL input, default filename, invalid ID, JSON output, url_private fallback)
• All existing tests pass
• Linter clean
• Live tested against real Slack file (F0AHF3NUSQK) with both file ID and URL input

Closes #121