Clean up stale repo artifacts

[changbowei]

Title:

Clean up stale repo artifacts

Body:

## Summary
- Move the queue gap type registry from `queue/` to `config/`
- Remove tracked backup/temp artifacts (`*.bak`, old runbook backups, simulation backup files)
- Remove the unused `src/simulation/engine_adr020.py`
- Consolidate the stray `tests/` test into the canonical `test/` tree
- Add ignore rules to prevent backup/temp/local tool files from being re-added

## Validation
- `uv run python -m src.cli queue gap-types`
- `uv run pytest -q test/unit/test_waste_support.py`
- `uv run pytest --collect-only -q`
- `uv run python -m src.cli index`

Note: full pytest was not run on this PR branch. Earlier full-suite testing on the local cleanup commit showed existing simulation behavior failures unrelated to this cleanup.