refactor: deduplicate agent hydration in classroom page load

[YizukiAme]

Problem

In app/classroom/[id]/page.tsx, the loadClassroom function hydrates server-generated agents twice via two independent code paths:

Path 1 (L55-62): Inline hydration after server fetch

if (stage.generatedAgentConfigs?.length) {
  const { saveGeneratedAgents } = await import('@/lib/orchestration/registry/store');
  const { useSettingsStore } = await import('@/lib/store/settings');
  const agentIds = await saveGeneratedAgents(stage.id, stage.generatedAgentConfigs);
  useSettingsStore.getState().setSelectedAgentIds(agentIds);
}

This writes agents to IndexedDB + Registry and sets selectedAgentIds.

Path 2 (L73-98): General agent restoration (always runs)

const generatedAgentIds = await loadGeneratedAgentsForStage(classroomId);
if (generatedAgentIds.length > 0) {
  useSettingsStore.getState().setAgentMode('auto');
  useSettingsStore.getState().setSelectedAgentIds(generatedAgentIds);
}

This clears all isGenerated agents from the Registry, re-loads them from IndexedDB (the same data Path 1 just wrote), and re-sets selectedAgentIds.

Result

The exact same agents are saved → cleared → re-loaded → re-selected. The operations are:

• saveGeneratedAgents(): clear IndexedDB + clear Registry + write IndexedDB + add to Registry
• loadGeneratedAgentsForStage(): clear Registry + read IndexedDB + add to Registry

Net effect is correct, but the clear-then-reload cycle is redundant.

Impact

Functional impact: none. The end state is identical. This is purely a code cleanliness issue — the double operation adds ~50ms of unnecessary IndexedDB I/O on classroom load.

Suggested Fix

Remove the setSelectedAgentIds call from Path 1 (L60), since Path 2 will handle it along with setAgentMode:

// Path 1: just persist to IndexedDB so Path 2 can find them
if (stage.generatedAgentConfigs?.length) {
  const { saveGeneratedAgents } = await import('@/lib/orchestration/registry/store');
  await saveGeneratedAgents(stage.id, stage.generatedAgentConfigs);
}

Context

Discovered during review of #336.