fix: nil Preamble on restored paused sessions

The persist path created agents with &Runtime{} (nil Preamble).
If a prompt arrived before resume rebuilt the runtime, Preamble.Set
panicked. Initialize with an empty Preamble.

cmd/olliesrv/internal/session/persist.go

@@ -204,7 +204,7 @@ func restoreMultiAgentSession(ps *PersistedSession) (*RestoredSession, error) {
 			history := agent.RestoreHistoryFromMessages(pa.Messages)
 			ag := agent.NewAgent(agent.AgentParams{
 				History:   history,
-				Runtime:   &agent.Runtime{},
+				Runtime:   &agent.Runtime{Preamble: &agent.Preamble{}},
 				Profile:   pa.Profile,
 				AgentsDir: pkgAgentsDir,
 				ID:        pa.ID,