Add decision persistence layer with DecisionRepository, DecisionModel,
and Alembic migration. Includes tree queries (BFS traversal via deque,
path-to-root), superseded lookup, ordered decision path retrieval,
concrete Decision type annotations (via TYPE_CHECKING), and comprehensive
test coverage (Behave BDD, Robot Framework, ASV benchmarks). Updated
database_schema.md, CHANGELOG.md, and CONTRIBUTORS.md.
ISSUES CLOSED: #171
khyari hamza