fix(todo-continuation-enforcer): prevent post-compaction agent fallback to General

After compaction, message history is truncated and the original agent
(e.g. Prometheus) can no longer be resolved from messages. The todo
continuation enforcer would then inject a continuation prompt with
agent=undefined, causing the host to default to General -- which has
write permissions Prometheus should never have.

Root cause chain:
1. handler.ts had no session.compacted handler (unlike Atlas)
2. idle-event.ts relied on finding a compaction marker in truncated
   message history -- the marker disappears after real compaction
3. continuation-injection.ts proceeded when agentName was undefined
   because the skipAgents check only matched truthy agent names
4. prometheus-md-only/agent-resolution.ts did not filter compaction
   agent from message history fallback results

Fixes:
- Add session.compacted handler that sets hasRecentCompaction state flag
- Replace fragile history-based compaction detection with state flag
- Block continuation injection when agent is unknown post-compaction
- Filter compaction agent in Prometheus agent resolution fallback

src/hooks/prometheus-md-only/agent-resolution.ts

@@ -12,21 +12,30 @@ import { isSqliteBackend } from "../../shared/opencode-storage-detection"
 
 type OpencodeClient = PluginInput["client"]
 
+function isCompactionAgent(agent: string): boolean {
+  return agent.toLowerCase() === "compaction"
+}
+
 async function getAgentFromMessageFiles(
   sessionID: string,
   client?: OpencodeClient
 ): Promise<string | undefined> {
   if (isSqliteBackend() && client) {
     const firstAgent = await findFirstMessageWithAgentFromSDK(client, sessionID)
-    if (firstAgent) return firstAgent
+    if (firstAgent && !isCompactionAgent(firstAgent)) return firstAgent
 
     const nearest = await findNearestMessageWithFieldsFromSDK(client, sessionID)
-    return nearest?.agent
+    if (nearest?.agent && !isCompactionAgent(nearest.agent)) return nearest.agent
+    return undefined
   }
 
   const messageDir = getMessageDir(sessionID)
   if (!messageDir) return undefined
-  return findFirstMessageWithAgent(messageDir) ?? findNearestMessageWithFields(messageDir)?.agent
+  const firstAgent = findFirstMessageWithAgent(messageDir)
+  if (firstAgent && !isCompactionAgent(firstAgent)) return firstAgent
+  const nearestAgent = findNearestMessageWithFields(messageDir)?.agent
+  if (nearestAgent && !isCompactionAgent(nearestAgent)) return nearestAgent
+  return undefined
 }
 
 /**