Add hierarchical decomposition with 4+ levels and bounded context per
subplan. Implement decomposition heuristics (max_files_per_subplan,
max_tokens_per_subplan, language/dir clustering). Add dependency closure
computation for large graphs and DAG execution ordering. Add bounded
dependency closure with cutoff thresholds and memoization for 10K+
files. Record decomposition decisions in DecisionService
(strategy_choice + subplan_spawn entries).
New modules:
- decomposition_models.py: DecompositionConfig, DecompositionNode,
DecompositionResult, DependencyEdge, DependencyGraph
- decomposition_clustering.py: ClusteringStrategy with directory,
language, and size clustering plus deterministic sort
- decomposition_graph.py: DependencyClosureComputer with bounded
closure, topological sort, cycle detection, and memoization
- decomposition_service.py: DecompositionService orchestrating
hierarchy building and decision recording
Settings: planner_max_depth, planner_max_files_per_subplan,
planner_max_tokens_per_subplan, planner_min_files_per_subplan
Closes#205
