v3.21.0 — visualization extracted to cortex-viz

• fix(build): drop stale ui/ force-include from the wheel after viz extraction
• release: v3.21.0 — extract visualization to the standalone cortex-viz MCP
• refactor: finish viz extraction — remove ui/ + viz skill, point docs to cortex-viz
• refactor: Phase 6 — strip the visualization stack from Cortex
• docs(readme): update version badge + What's new for v3.20.0

Full Changelog: v3.20.0...v3.21.0