feat(sessions): add SessionAdapter — REST CRUD for sessions/milestones/episodes (#50)

New internal/sessions package with Handler bridging 7 REST endpoints to:
- POST /memory/ingest on poimen-memory:8080 (writes)
- GraphQL hybridSearch/entity on poimen-graphql:8090 (reads)

Adapter registration: X-Service: sessions with resources sessions,
session, milestones, episodes, episode. IDs via query params
(?session_id=, ?episode_id=) — dispatcher does not substitute path params.

episode_id = sha256(session_id+milestone_id+attempt) per §4.14 Q6.
Content JSON-encoded (prevents key:value injection on user fields).
Limit capped at 200. GraphQL error body detected and surfaced as 502.
tracing.NewTransport wired for cross-service trace propagation.
JWT forwarded to both upstreams for auth.

30 tests in handler_test.go covering all 7 handlers + EpisodeID helper.

Closes #50

internal/serviceadapter/adapters.go

@@ -28,6 +28,9 @@ func AdapterFactory(serviceName string) interface{} {
 		return &IAMAdapter{}
 	case "memory":
 		return &MemoryAdapter{}
+	case "sessions":
+		// code-registered; see NewSessionAdapter()
+		return nil
 	default:
 		return nil
 	}