- Create ArtifactPill component (similar to SkillPill)
- Renders filename as clickable pill with FileCode2 icon
- Clicking pill opens artifacts panel and selects that artifact
- Update ArtifactsToolRenderer to accept artifactsPanel reference
- Pass artifactsPanel from ChatPanel to renderer on initialization
- Display artifact pill below header for all commands
- Pill only clickable when artifactsPanel reference is available
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