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/main.ts

@@ -481,8 +481,15 @@ async function main() {
       },
     });
     
-    // Verify agent exists (clear stale ID if deleted)
+    // Apply explicit agent ID from config (before store verification)
     let initialStatus = bot.getStatus();
+    if (agentConfig.id && !initialStatus.agentId) {
+      console.log(`[Agent:${agentConfig.name}] Using configured agent ID: ${agentConfig.id}`);
+      bot.setAgentId(agentConfig.id);
+      initialStatus = bot.getStatus();
+    }
+    
+    // Verify agent exists (clear stale ID if deleted)
     if (initialStatus.agentId) {
       const exists = await agentExists(initialStatus.agentId);
       if (!exists) {