Artifact tool: allow setting sharing/visibility programmatically

Feature request

The Artifact tool can publish and redeploy artifacts, but there is no way for the agent to change an artifact's sharing setting – every artifact is private to the author until a human opens the artifact page and flips the share control.

Concrete workflow this blocks

In a Claude Code session today the agent:

1. Investigated a production bug and raised a PR
2. Generated a commit-by-commit reviewer walkthrough as an artifact (quoted source, per-commit narrative, test coverage map)
3. Edited the PR description to lead with a link to that artifact for the review team

Step 4 – "make it available to the whole team" – could not be completed by the agent. The session had to stop and hand back to the human for one click in the artifact UI, and until then the PR opened with a link teammates couldn't access.

We expect to use artifacts heavily in exactly this way (PR walkthroughs, investigation write-ups, run-books linked from tickets), so the manual visibility step becomes a recurring papercut in otherwise end-to-end agent workflows.

Suggestion

A visibility (or share) parameter on the Artifact tool – e.g. private (default) / organisation – or a companion tool for managing sharing on artifacts the user owns. Keeping private as the default preserves the current safe behaviour; the agent would only widen visibility when the user asks for it, as they did here.