feat(learning): add learning runtime with pattern mining

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/app/agents/learning/audit.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from app.agents.schemas.learning import LearningDecision, SessionRetrospective
+
+
+def build_learning_audit_entry(retrospective: SessionRetrospective) -> dict[str, object]:
+    decision = retrospective.learning_decision
+    return {
+        "retrospective_id": retrospective.retrospective_id,
+        "decision": decision.decision if isinstance(decision, LearningDecision) else None,
+        "explanation": decision.explanation if isinstance(decision, LearningDecision) else None,
+        "signal_count": len(retrospective.learning_signals),
+        "pattern_count": len(retrospective.pattern_candidates),
+        "skill_candidate_count": len(retrospective.skill_candidates),
+        "outcome": retrospective.outcome,
+    }