Add execution engine and task lifecycle services

docs/backlog/01_execution_backlog.md

@@ -18,13 +18,13 @@
 
 ## Package B: Core Hardening
 
-- [ ] graph scheduler service
-- [ ] node runner abstraction
+- [x] graph scheduler service
+- [x] node runner abstraction
 - [ ] reviewer contract
-- [ ] finalizer contract
-- [ ] resume/cancel use cases
-- [ ] idempotency policy
-- [ ] retry policy matrix
+- [x] finalizer contract
+- [x] resume/cancel use cases
+- [x] idempotency policy
+- [x] retry policy matrix
 
 ## Package C: Persistence
 
@@ -57,4 +57,3 @@
 - [ ] scenario tests
 - [ ] smoke suite
 - [ ] static checks in CI
-