feat(sideshow-term): add clear command

[benvinegar]

Summary

• Add sideshow-term clear for agents to remove existing visualizations from the current session
• Add sideshow-term clear --all to clear every session on the surface, plus --session and --sessions controls
• Document the clear command in the README, setup block, skill, and changelog

Validation

• node --check bin/sideshow-term.js
• npm test
• npm run test:render
• npm run build
• npx oxlint sideshow-term/bin/sideshow-term.js --deny-warnings
• Smoke-tested sideshow-term demo followed by sideshow-term clear --all against a temp server/data file

This PR description was generated by Pi using OpenAI GPT-5