fix(hub): resume Claude SDK sessions

hub/prisma/schema.prisma

@@ -3,9 +3,9 @@
 // Aligns to spec/System (ADR-0001..0004, 0017). Key divergences from the
 // legacy teaching-material-host-service schema, each deliberate:
 //
-//  - No AgentSession.claudeSessionId. ADR-0017: session is provider/model-
-//    bound, not Claude-bound; `provider`+`model` replace it. Switching model
-//    = new session, so the unique constraint is on the session id alone.
+//  - AgentSession is provider/model-bound. Provider runtime cursors such as
+//    Claude SDK `session_id` live in `metadata`, so switching model still means
+//    a new Hub session while same-session runs can resume provider context.
 //  - PermissionRole enum = read/edit/manage (ADR-0004 capability lattice),
 //    distinct from platform UserRole (admin/teacher) — legacy conflated them.
 //  - ProjectGroupBinding is project→chat only (ADR-0001 1:1); legacy mixed
@@ -110,9 +110,10 @@ model ProjectGroupBinding {
 
 // --- AgentRun, session, lock (ADR-0002, 0017) -----------------------------
 
-/// ADR-0017: session is provider/model-bound. No claudeSessionId; `provider`
-/// + `model` capture the binding. Same provider+model ⇒ reuse across runs
-/// (ADR-0002); a switch ⇒ new session.
+/// ADR-0017: session is provider/model-bound. `provider` + `model` capture
+/// the binding; provider-specific runtime cursors live in `metadata`
+/// (e.g. metadata.claudeSessionId). Same provider+model ⇒ reuse across runs
+/// (ADR-0002); a switch ⇒ new Hub session.
 model AgentSession {
   id         String    @id @default(cuid())
   projectId  String