chore: prepare 0.20.0 release

[douglasmonsky]

Summary

• bump package, plugin, and MCP runtime versions to 0.20.0
• publish the 0.20.0 changelog covering inline thread expansion, Simplified Chinese localization, visualization layout fixes, and long-context pricing
• align public release verification documentation with 0.20.0

Validation

• 1,013 tests passed with 88% coverage
• Ruff, mypy, compileall, dashboard JavaScript syntax, and dashboard verification passed
• release readiness and whitespace checks passed
• wheel and sdist built; Twine and distribution checks passed
• clean installed-package smoke passed