Implement SubplanExecutionService and SubplanMergeService to enable
parent plans to decompose work into coordinated child subplans with
configurable execution modes (sequential, parallel, dependency-ordered)
and merge strategies (git_three_way, sequential_apply, fail_on_conflict,
last_wins).
- SubplanExecutionService: schedules subplan execution with retry
support via SubplanFailureHandler, max_parallel limits, fail_fast
semantics, and topological DAG ordering for dependency mode
- SubplanMergeService: wraps sandbox merge infrastructure to combine
subplan sandbox outputs using the configured merge strategy
- BDD tests: 21 scenarios covering all execution modes, merge
strategies, validation, and integration flows
- Robot tests: 11 integration test cases with helper module
- ASV benchmarks: performance benchmarks for execution and merge
- Documentation: reference guide in docs/reference/subplans.md
ISSUES CLOSED: #184
Luis Mendes