session-memory Phase 0: normalized schema (T01) + Claude adapter (T02)

- session_memory/core/schema.py: Session/SessionEvent/Cost dataclasses,
  flavor-prefixed uids, watermarks, kind/outcome validation (T01)
- session_memory/adapters/claude.py: JSONL -> Normalized bundle, turn DAG
  via uuid/parentUuid, kind mapping, cost from message.usage (T02)
- tests: schema round-trip + adapter (synthetic + real local session)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

session_memory/__init__.py

@@ -0,0 +1,7 @@
+"""Coding Session Memory — Helix Forge capture + retention layer.
+
+See docs/DESIGN-session-memory.md. Importable package name uses an underscore
+(``session_memory``) where the design doc writes ``session-memory/``.
+"""
+
+__all__ = ["core", "adapters"]