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/__init__.py

@@ -0,0 +1,19 @@
+from app.agents.learning.jobs import persist_retrospective, schedule_retrospective_job
+from app.agents.learning.pattern_miner import LearningPatternMiner
+from app.agents.learning.retrospector import build_session_retrospective
+from app.agents.learning.session_search import SessionRetrospectiveSearch
+from app.agents.learning.signal_extractor import RetrospectiveSignalExtractor
+from app.agents.learning.skill_candidate_builder import SkillCandidateBuilder
+from app.agents.learning.store import LearningArtifactStore, SessionRetrospectiveStore
+
+__all__ = [
+    "build_session_retrospective",
+    "LearningArtifactStore",
+    "LearningPatternMiner",
+    "persist_retrospective",
+    "RetrospectiveSignalExtractor",
+    "schedule_retrospective_job",
+    "SessionRetrospectiveSearch",
+    "SessionRetrospectiveStore",
+    "SkillCandidateBuilder",
+]