feat: agent session management — metrics, channels, dispose (P2-006) (#78)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/agent/session.dto.ts

@@ -0,0 +1,14 @@
+export interface SessionInfoDto {
+  id: string;
+  provider: string;
+  modelId: string;
+  createdAt: string;
+  promptCount: number;
+  channels: string[];
+  durationMs: number;
+}
+
+export interface SessionListDto {
+  sessions: SessionInfoDto[];
+  total: number;
+}