feat: add diagnostic dashboard snapshots

[douglasmonsky]

Summary

• add persisted aggregate diagnostic snapshots and on-demand refresh endpoints
• add diagnostics reports for tool output, commands, file reads, read productivity, and concentration
• isolate diagnostics from normal live dashboard refresh and add expandable command children
• add Playwright diagnostics smoke coverage and release documentation updates

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
• DASHBOARD_BASE_URL=http://127.0.0.1:8898 npm run smoke:dashboard:diagnostics

Privacy

• diagnostic snapshots persist aggregate rows only
• no raw prompts, assistant text, tool output, command text, patch text, or raw source-log paths are stored in snapshot rows