fix: multi-agent state isolation and config.id wiring (#218)

Two bugs from the Phase 1 merge:

1. Store not scoped to agent name: LettaBot constructor passed no agent
   name to Store, so all agents in multi-agent mode shared the same
   agentId/conversationId state. Now passes config.agentName.

2. agentConfig.id ignored: Users could set `id: agent-abc123` in YAML
   but it was never applied. Now checked before store verification.

Written by Cameron ◯ Letta Code

"The best error message is the one that never shows up." -- Thomas Fuchs

src/core/bot.ts

@@ -101,7 +101,7 @@ export class LettaBot implements AgentSession {
     mkdirSync(config.workingDir, { recursive: true });
     
     // Store in project root (same as main.ts reads for LETTA_AGENT_ID)
-    this.store = new Store('lettabot-agent.json');
+    this.store = new Store('lettabot-agent.json', config.agentName);
     
     console.log(`LettaBot initialized. Agent ID: ${this.store.agentId || '(new)'}`);
   }