[FEATURE] Cowork artifacts should not be auto-pinned; add option to create artifacts unpinned

Summary

All artifacts created in Cowork sessions are automatically pinned in the sidebar. The create_artifact tool has no pinned parameter, so the agent has no way to create an artifact unpinned.

Problem

In automated wrap workflows, Claude creates multiple artifacts per session — handoff briefs, summaries, etc. All are pinned regardless of whether user action is needed, creating sidebar clutter that requires manual cleanup after every session.

Proposed Fix

1. Add a pinned: boolean parameter to create_artifact (default: false). Agent sets pinned: true only when the artifact requires user action before the next session.
2. Add a UI pin/unpin toggle on each artifact card so users can manage this manually.

Related Issues

• #57177 - configurable Cowork workspace base path
• #54138 - hardcoded artifacts storage path

Workaround

None currently. Users must manually unpin after every session.