chore: prepare 0.9.0 release

[douglasmonsky]

Summary

• bump package, plugin manifest, and bundled MCP runtime pin to 0.9.0
• add 0.9.0 changelog notes for diagnostic snapshots and dashboard panels
• update release docs and public smoke command references to 0.9.0

Validation

• .venv/bin/python -m ruff check .
• .venv/bin/python -m mypy
• .venv/bin/python -m pytest
• .venv/bin/python -m pytest --cov=codex_usage_tracker --cov-report=term-missing
• .venv/bin/python -m compileall src
• for file in src/codex_usage_tracker/plugin_data/dashboard/dashboard*.js; do node --check "$file"; done
• .venv/bin/python scripts/check_release.py
• git diff --check
• rm -rf dist build src/codex_usage_tracker.egg-info src/codex_usage_tracking.egg-info && .venv/bin/python -m build
• .venv/bin/python -m twine check dist/*
• .venv/bin/python scripts/check_release.py --dist
• .venv/bin/python scripts/smoke_installed_package.py
• .venv/bin/python scripts/smoke_installed_package.py --docker
• DASHBOARD_BASE_URL=http://127.0.0.1:8898 npm run smoke:dashboard:diagnostics

Release notes

0.9.0 adds persisted aggregate diagnostic snapshots, on-demand diagnostics refresh, tool/command/file-read/concentration reports, and Diagnostics dashboard panels while preserving the aggregate-only privacy boundary.