fix: persist agent configs in server-generated classrooms

[YizukiAme]

Summary

Fix server-generated classrooms silently skipping all discussion actions due to missing agent configuration persistence.

Root Cause

The server-side classroom generation pipeline (/api/generate-classroom) generates agent profiles and passes them to scene/action generation, but never embeds them in the persisted Stage object. When a client loads the classroom:

1. loadGeneratedAgentsForStage() queries IndexedDB → empty (server can't write to client IndexedDB)
2. selectedAgentIds remains empty
3. The playback engine's isAgentSelected() guard returns false for every discussion action
4. All discussions are silently skipped via consumedDiscussions.add() + processNext()

Changes

1. lib/types/stage.ts

• Added optional generatedAgentConfigs field to Stage interface
• Type shape matches saveGeneratedAgents() input exactly for zero-transform pass-through

2. lib/server/classroom-generation.ts

• Added AGENT_COLORS and AGENT_AVATARS constants (consistent with /api/generate/agent-profiles)
• Injected generatedAgentConfigs into Stage object during construction with fallback avatar/color/priority

3. app/classroom/[id]/page.tsx

• After loading classroom from server storage, hydrates stage.generatedAgentConfigs into IndexedDB + agent registry via existing saveGeneratedAgents()
• Sets selectedAgentIds so the playback engine's isAgentSelected() guard passes correctly

Data Flow (After Fix)

Server: generateAgentProfiles() → agents[]
  ↓
Stage.generatedAgentConfigs = agents (with avatar/color/priority)
  ↓
persistClassroom() → data/classrooms/{id}.json
  ↓
Client: fetch /api/classroom → stage.generatedAgentConfigs
  ↓
saveGeneratedAgents() → IndexedDB + Registry
  ↓
setSelectedAgentIds() → Settings Store
  ↓
isAgentSelected(agentId) → ✅ true → Discussion plays!

Backward Compatibility

• generatedAgentConfigs is optional — existing classroom JSON files load normally
• Frontend generation-preview flow is unaffected (uses IndexedDB directly, never touches this field)
• Old server-generated classrooms (without this field) behave as before (no regression)

Verification

• pnpm exec tsc --noEmit → exit code 0, zero errors

Closes #316

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Assisted by Claude Opus 4.6.