Add SubplanService for building child plans from DecisionService spawn
entries and SubplanConfig. The service validates resource scopes, merge
strategies, and max_parallel bounds before spawning.
Key additions:
- SubplanService: Orchestrates child plan creation from spawn entries
- SpawnMetadata: Persisted metadata (spawn_decision_id, parent/root
plan IDs, execution mode) for status output
- Spawn validation: Checks resource scopes, merge strategy, and
parallelism bounds before spawning
- Documentation: subplan_service.md with spawn workflow and lifecycle
ISSUES CLOSED: #197
