feat(memory): add episodic memory implementation (Issue #90)

Implements the episodic memory service for storing and retrieving
agent task execution experiences. This enables learning from past
successes and failures.

Components:
- EpisodicMemory: Main service class combining recording and retrieval
- EpisodeRecorder: Handles episode creation, importance scoring
- EpisodeRetriever: Multiple retrieval strategies (recency, semantic,
  outcome, importance, task type)

Key features:
- Records task completions with context, actions, outcomes
- Calculates importance scores based on outcome, duration, lessons
- Semantic search with fallback to recency when embeddings unavailable
- Full CRUD operations with statistics and summarization
- Comprehensive unit tests (50 tests, all passing)

Closes #90

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/memory/episodic/__init__.py

@@ -1,8 +1,17 @@
+# app/services/memory/episodic/__init__.py
 """
-Episodic Memory
+Episodic Memory Package.
 
-Experiential memory storing past task completions,
-failures, and learnings.
+Provides experiential memory storage and retrieval for agent learning.
 """
 
-# Will be populated in #90
+from .memory import EpisodicMemory
+from .recorder import EpisodeRecorder
+from .retrieval import EpisodeRetriever, RetrievalStrategy
+
+__all__ = [
+    "EpisodeRecorder",
+    "EpisodeRetriever",
+    "EpisodicMemory",
+    "RetrievalStrategy",
+]