Support creating native YouTube embeds in rich-text documents, or document the API/SDK prerequisite #869

Use case

I use the CLI to maintain a library of source notes in Docs & Files. Each document
summarizes one video and links back to the original. I would like to insert the
same native YouTube player offered by the web editor, instead of uploading a
static thumbnail and adding a separate link.

Observed behavior

• Installed version: basecamp version 0.13.0, on macOS.
• basecamp files documents create and basecamp files update support rich text.
  HTML formatting and inline image attachments work for this workflow.
• I could not find a native external-media/embed creation command in the installed
  command help.
• Pasting a YouTube URL into the web editor offers Yes, embed it. Selecting it
  produces an embedded video preview.
• As a diagnostic, basecamp api post embeds.json with a JSON url pointing to
  a public YouTube video returned not_found / 404 on the configured API host.
• Passing the equivalent absolute URL on app.basecamp.com to basecamp api post
  was rejected because it was not on the configured host. That credential
  destination restriction is appropriate and should remain in place.

The two API probes above do not establish that native embeds are impossible
through every API route, or that the missing capability belongs solely to the
CLI. They establish the limit of the route I tried.

How to observe the gap

1. On CLI 0.13.0, inspect basecamp files documents create --agent --help,
   basecamp files update --agent --help, and the available command catalogue.
2. In a document's web editor, paste a public YouTube URL on its own line and
   choose Yes, embed it.
3. Attempt to find the supported CLI equivalent for producing that native embed
   under the same authorized identity, without reusing browser cookies.

I have not tested an undocumented HTML encoding as a substitute, nor verified
that updating an existing native player through the CLI preserves playback.

Requested outcome

Please clarify whether the public API and Go SDK support creating an external
video embed from a URL. If they do, a CLI operation returning a rich-text
attachment reference would make this workflow possible. The exact command name
and interface are left to the maintainers.

Useful behavior would include:

• Creating an embed under the selected account/profile through a supported API.
• Returning a reference usable by document creation and update commands.
• Handling pending, unsupported or unavailable media with structured output.
• Preserving an existing embed when editing surrounding document text.
• Documenting the distinction between an inline image, a native video embed and
  the document's appearance in the Docs & Files listing.

If this requires API or SDK work first, a linked prerequisite or documentation
of the limitation would already help. I am not requesting a browser-session
workaround or weakening the configured-host restriction.

Contribution context

I reviewed CONTRIBUTING.md and the SDK-first guidance in AGENTS.md before preparing
this request. This is a feature/capability question, not a claim of a confirmed
CLI regression. No code change or untested implementation is proposed here.