fix(background-agent): retry with fallback agent on Agent not found error

When a model/mode switch happens while a background task is in-flight,
the oh-my-openagent agent registry can be rebuilt without custom agents
(e.g., Sisyphus-Junior). The SDK then rejects the promptAsync call with
"Agent not found", killing the task.

This adds retry logic: when promptAsync fails with "Agent not found",
retry with the "general" agent (always available in opencode). The
original prompt, model, and skill content are preserved — only the
agent routing changes.

Fixes both the spawner (startTask/resumeTask) and manager (inline
launch) code paths. Also improves the error message detection in
manager.ts to recognize "Agent not found" alongside the existing
"agent.name"/"undefined" checks.

Related: #2052, #2875, #2882

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/features/background-agent/manager.ts

@@ -1,5 +1,6 @@
 
 import type { PluginInput } from "@opencode-ai/plugin"
+import { isAgentNotFoundError } from "./spawner"
 import type {
   BackgroundTask,
   LaunchInput,
@@ -546,32 +547,54 @@ export class BackgroundManager {
       applySessionPromptParams(sessionID, input.model)
     }
 
+    const FALLBACK_AGENT = "general"
+
+    const promptBody = {
+      agent: input.agent,
+      ...(launchModel ? { model: launchModel } : {}),
+      ...(launchVariant ? { variant: launchVariant } : {}),
+      system: input.skillContent,
+      tools: (() => {
+        const tools = {
+          task: false,
+          call_omo_agent: true,
+          question: false,
+          ...getAgentToolRestrictions(input.agent),
+        }
+        setSessionTools(sessionID, tools)
+        return tools
+      })(),
+      parts: [createInternalAgentTextPart(input.prompt)],
+    }
+
     promptWithModelSuggestionRetry(this.client, {
       path: { id: sessionID },
-      body: {
-        agent: input.agent,
-        ...(launchModel ? { model: launchModel } : {}),
-        ...(launchVariant ? { variant: launchVariant } : {}),
-        system: input.skillContent,
-        tools: (() => {
-          const tools = {
-            task: false,
-            call_omo_agent: true,
-            question: false,
-            ...getAgentToolRestrictions(input.agent),
-          }
-          setSessionTools(sessionID, tools)
-          return tools
-        })(),
-        parts: [createInternalAgentTextPart(input.prompt)],
-      },
+      body: promptBody,
     }).catch(async (error) => {
+      // Retry with fallback agent if the original agent was unregistered (e.g., after a model switch)
+      if (isAgentNotFoundError(error) && input.agent !== FALLBACK_AGENT) {
+        log("[background-agent] Agent not found, retrying with fallback agent", {
+          original: input.agent,
+          fallback: FALLBACK_AGENT,
+          taskId: task.id,
+        })
+        try {
+          await promptWithModelSuggestionRetry(this.client, {
+            path: { id: sessionID },
+            body: { ...promptBody, agent: FALLBACK_AGENT },
+          })
+          return
+        } catch (retryError) {
+          log("[background-agent] Fallback agent also failed:", retryError)
+        }
+      }
+
       log("[background-agent] promptAsync error:", error)
       const existingTask = this.findBySession(sessionID)
       if (existingTask) {
         existingTask.status = "interrupt"
         const errorMessage = error instanceof Error ? error.message : String(error)
-        if (errorMessage.includes("agent.name") || errorMessage.includes("undefined")) {
+        if (errorMessage.includes("agent.name") || errorMessage.includes("undefined") || isAgentNotFoundError(error)) {
           existingTask.error = `Agent "${input.agent}" not found. Make sure the agent is registered in your opencode.json or provided by a plugin.`
         } else {
           existingTask.error = errorMessage