Add aggregate diagnostics dashboard

[douglasmonsky]

Summary

• Persist aggregate diagnostic facts for parser, tool, lifecycle, compaction, and outcome signals without storing raw transcript content
• Add diagnostics CLI/API surfaces and a dashboard diagnostics tab with sortable/pageable associated calls
• Harden diagnostics loading, no-op refresh behavior, and context evidence for lifecycle facts such as thread rollbacks

Privacy

• Diagnostic facts remain aggregate metadata only
• Raw content is not persisted to SQLite
• Evidence remains explicit, on-demand, redacted, and size-limited

Validation

• python -m ruff check .
• python -m mypy
• python -m pytest
• python -m pytest --cov=codex_usage_tracker --cov-report=term-missing
• python -m compileall src
• node --check dashboard assets
• python scripts/check_release.py
• python -m build
• python -m twine check dist/*
• python scripts/check_release.py --dist
• git diff --check

Release note

• Candidate scope for 0.8.0 after this merges to main and a release prep branch bumps metadata/changelog.