Fix cfgit read-path footguns

[AusafMo]

Summary

• remove the silent dev fallback from env:DEV_MONGODB_URI to generic MONGODB_URI
• keep sequence refs explicitly sigiled as @N, but make bare numeric ref misses point users to @N
• bump cfgit and cfgit-impact to 0.1.3 for the next PyPI release

Why

The read path was not broken. The confusing behavior came from two footguns: missing DEV_MONGODB_URI could silently point cfgit at a different database via MONGODB_URI, and typing 12 instead of @12 produced a technically correct but unhelpful ref error.

Validation

• python -m pytest tests/test_config.py tests/test_engine_safety.py tests/test_mongo_history.py tests/test_postgres_history.py -q
• python -m pytest -q twice
• ruff check ...
• git diff --check
• built both wheels as 0.1.3
• twine check on both wheels