No tool for creating an upload — replace_upload has no counterpart

#4 added replace_upload (publish a new version of an existing upload) and list_upload_versions. There is still no tool that creates an upload, so the versioning story only works on files that got into Basecamp some other way.

Today: basecamp_run ["files", "uploads", "create", "<path>", "--project", "<id>"], documented in AGENTS.md.

Why this was left out rather than added

AGENTS.md's scope policy: parity with the official Basecamp skill is the baseline, and anything beyond it is an explicit decision, not something added opportunistically while nearby. The official skill covers Docs & Files reads only; create_document / update_document / replace_upload are already explicit extensions. upload_file would be one more, and it is worth deciding on its own rather than as a side effect of the v0.9.1 pass.

If added

basecamp files uploads create <file> flags:

• --project / --vault (folder, defaults to root)
• --description (Markdown)
• --visible-to-clients — root Docs & Files folder only; a nested folder inherits its folder's visibility. Omit for the server default, so this needs three states (true / false / unset), not a boolean that defaults to false.

Note the CLI does it in two steps: the file is uploaded as an attachment, then created as an upload in the target vault. A failure between the two leaves a stranded attachment.

Start by reading AGENTS.md and comparing the existing replace_upload and list_upload_versions tools with the documented basecamp files uploads create command. Clarify whether upload_file belongs in scope, then trace the CLI's attachment-then-upload flow and account for the listed flags and stranded-attachment failure case; done means the scope decision and behavior are documented and implemented if approved.