宜博 / lemongraph

Implements docs/sdd/01-M-模型设计.md §1.0-1.14 as stdlib sqlite3 DDL in src/
(deployment/deploy_health_signal §1.15-1.16 are PH5/U-034, deliberately
excluded). Covers all entities' fields, enum CHECK constraints, FKs, and
the two non-trivial unique indexes (work_graph_run's per-(task_id,org_node)
active-run partial index, artifact's (task_id,artifact_type,git_commit_sha)
composite key).

Per user decision (2026-07-30): bug_record.root_cause_category excludes
'core_engine' for now -- that value is PH5-scoped and development-plan.md
assigns adding it to U-034, not U-001; a test locks this in.

14 stdlib unittest cases in src/test_schema.py cover table creation and the
Done criterion's named constraint classes (unique/FK/enum), all passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>