Add support for archiving projects

[roman-marszalek]

Would love to see basecamp project archive <id> (and ideally unarchive) added to the CLI.

Currently, archiving requires navigating the Basecamp web UI, which breaks the flow when managing projects from the terminal or via an AI agent. Being able to archive projects programmatically would make bulk cleanup and agent-driven workflows much more practical.

Note: the underlying API doesn't appear to expose an archive endpoint for projects either, so this may require an API addition first, but flagging it here as the desired CLI capability.