Implement MetricsStore for project-scoped agent metrics.

Add ADR-004 storage layer with append-only executions, summary
regeneration, idempotency keys, and retention pruning. Wire memory init
to scaffold .kaizen/metrics/ by default and add unit tests.

src/kaizen_agentic/__init__.py

@@ -16,6 +16,7 @@ from .core import Agent, AgentConfig
 from .optimization import OptimizationLoop, PerformanceMetrics
 from .registry import AgentRegistry, AgentDefinition, AgentCategory
 from .installer import AgentInstaller, ProjectInitializer, InstallationConfig
+from .metrics import MetricsStore
 
 __all__ = [
     "Agent",
@@ -28,4 +29,5 @@ __all__ = [
     "AgentInstaller",
     "ProjectInitializer",
     "InstallationConfig",
+    "MetricsStore",
 ]