Publish HTML directly from the OpenArtifacts skill

Context

Copilot's built-in OpenArtifacts skill publishes from Markdown only. The official OpenArtifacts npm CLI already accepts an HTML file directly.

The Markdown-only path was a deliberate choice from when Obsidian could not open a local HTML file, so there was nothing for a user to preview. Agent Mode has terminal access and can call the CLI itself, so that constraint no longer decides what the skill can publish.

Agents convert Markdown to HTML well, and a user who has had an agent produce HTML currently has no way to publish it through Copilot.

Reported in https://discord.com/channels/1182847556032659546/1548703495979270154.

User experience

Today an agent can produce an HTML artifact and then cannot publish it, even though the CLI sitting on the same machine would accept it. The user either drops back to the command line themselves or converts back to Markdown and loses the formatting that made HTML worth producing.

Afterwards the built-in skill publishes an HTML file as it stands.

Proposed behavior

• The OpenArtifacts skill publishes an HTML file directly rather than only Markdown.
• Markdown publishing keeps working unchanged.
• Publishing HTML returns the same page link and receipt as publishing Markdown.

Explicit non-goals

• Rendering or previewing local HTML inside Obsidian.
• Password-protected or otherwise access-restricted published links. Raised in the same thread and tracked separately if the team decides to take it on.