feat(sessions): SessionAdapter — REST CRUD for sessions/milestones/episodes (#51)

Closes #50

## Changes

- `internal/sessions/handler.go` — new Handler; 7 endpoints dispatched via `X-Upstream-Path`; writes to `poimen-memory` via `/memory/ingest`; reads from `poimen-graphql` via GraphQL `hybridSearch`/`entity`
- `internal/serviceadapter/session_adapter.go` — `NewSessionAdapter()` + `GetSessionSpec()` with 5 resources
- `cmd/gateway/main.go` — register sessions adapter after memory adapter
- `internal/serviceadapter/adapters.go` — add `sessions` case to factory
- `internal/sessions/handler_test.go` — 30 tests, all pass

## Design

- `X-Service: sessions` + `X-Resource: <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 (injection-safe)
- `tracing.NewTransport` wired for cross-service trace propagation
- GraphQL `errors` array detected, surfaced as 502
- limit capped at 200

---------

Co-authored-by: Admin Bot <[email protected]>
Reviewed-on: #51

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
 	}