chore: prepare 0.21.0 release

[douglasmonsky]

Summary

• bump the package, plugin, and bundled MCP runtime to 0.21.0
• publish release notes for the persistent dashboard service, polished thread expansion, Fast/tier-aware usage tracking, and Windows filesystem ID compatibility
• update public development and readiness examples to the new version

Verification

• python -m ruff check .
• python -m mypy
• python -m pytest — 1114 passed
• python -m pytest --cov=codex_usage_tracker --cov-report=term-missing — 1114 passed, 88% total coverage
• python -m compileall src
• node --check for every bundled dashboard JavaScript file
• python scripts/check_release.py
• git diff --check main...HEAD
• python -m build
• python -m twine check dist/*
• python scripts/check_release.py --dist
• python scripts/smoke_installed_package.py

Release

After this PR is merged, create the v0.21.0 GitHub release from updated main and verify the Trusted Publishing workflow and public package.