fix: `generatedAgentConfigs` unconditionally set — default agents polluted as "generated"

[YizukiAme]

Problem

In lib/server/classroom-generation.ts, generatedAgentConfigs is set on the Stage object unconditionally — regardless of whether agentMode is 'default' or 'generate':

// L216-227: agents resolved based on agentMode
if (agentMode === 'generate') {
  agents = await generateAgentProfiles(requirement, lang, aiCall);
} else {
  agents = getDefaultAgents();  // returns default-1 ~ default-6
}

// L306: unconditional — always set, even for default agents
generatedAgentConfigs: agents.map((a, i) => ({ ... }))

When agentMode === 'default', generatedAgentConfigs contains the 6 default agents (default-1 ~ default-6).

On the client side, saveGeneratedAgents() saves them to IndexedDB with isGenerated: true, overwriting the original isDefault: true entries in the Agent Registry.

Consequence

If a user navigates from a server-generated (default-mode) classroom to another classroom in the same SPA session:

1. loadGeneratedAgentsForStage(newClassroomId) clears all isGenerated agents from the Registry
2. The default agents (default-1 ~ default-6) were overwritten as isGenerated in step 1 of the previous load — so they get deleted
3. The original isDefault entries are gone (only restored on full page refresh via zustand persist merge)

Practical impact is low (page refresh fixes it), but it's a latent correctness issue.

Related: Stage missing agentIds for default mode

When using default agents, the Stage object also doesn't set the agentIds field. The client-side fallback in page.tsx (L96) defaults to ['default-1', 'default-2', 'default-3'] — only 3 of the 6 defaults. This is currently masked by the unconditional generatedAgentConfigs, but would become visible if the above is fixed.

Suggested Fix

const stage: Stage = {
  // ...
  // Only embed agent configs for LLM-generated agents
  ...(agentMode === 'generate' ? {
    generatedAgentConfigs: agents.map((a, i) => ({
      id: a.id, name: a.name, role: a.role,
      persona: a.persona || '',
      avatar: AGENT_DEFAULT_AVATARS[i % AGENT_DEFAULT_AVATARS.length],
      color: AGENT_COLOR_PALETTE[i % AGENT_COLOR_PALETTE.length],
      priority: a.role === 'teacher' ? 10 : a.role === 'assistant' ? 7 : 5,
    }))
  } : {
    agentIds: agents.map(a => a.id)  // default mode: just record IDs
  }),
};

Context

Discovered during review of #336. The unconditional setting was introduced in that PR.