Support uploading a new version of an existing file

[olivaresf]

Problem

When an upload already exists in Docs & Files, there's no way to add a new version via the CLI. The Basecamp 3 API supports this (PUT to /buckets/{project}/uploads/{id}.json with a new attachable_sgid), but the CLI only has files uploads create which always creates a new upload.

Use case

We have a release script that signs and uploads Windows .exe builds to a Basecamp vault for internal testing. Each new build should replace the previous one as a new version of the same upload, not create a duplicate file.

Proposed solution

Add a --replace <id|url> flag to files uploads create, or a new command like files uploads update <id|url> <file>, that:

1. Uploads the file as an attachment (existing two-step flow)
2. PUTs to the existing upload with the new attachable_sgid

This keeps version history intact in Basecamp rather than creating separate uploads.