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

cmd/gateway/main.go

@@ -117,6 +117,10 @@ func main() {
 	// Includes standing-context resource for CAG support (issue #44).
 	_ = registry.Add(serviceadapter.NewMemoryAdapter())
 
+	// Add sessions service adapter — REST CRUD bridged to poimen-memory ingest
+	// and poimen-graphql reads. Issue #50.
+	_ = registry.Add(serviceadapter.NewSessionAdapter())
+
 	// Add other adapters from config (skip if already registered in code)
 	for _, a := range cfg.Adapters {
 		if existing := registry.Get(a.ServiceName); existing != nil {