fix(background-task): gracefully handle agent not found errors

When an invalid or unregistered agent is passed to background_task or
call_omo_agent, OpenCode crashes with "TypeError: undefined is not an
object (evaluating 'agent.name')". This fix:

- Validates agent parameter is not empty before launching
- Catches prompt errors and returns friendly error message
- Notifies parent session when background task fails
- Improves error message to guide user on resolution

Fixes #37

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/call-omo-agent/tools.ts

@@ -114,17 +114,26 @@ async function executeSync(
   log(`[call_omo_agent] Sending prompt to session ${sessionID}`)
   log(`[call_omo_agent] Prompt text:`, args.prompt.substring(0, 100))
 
-  await ctx.client.session.prompt({
-    path: { id: sessionID },
-    body: {
-      agent: args.subagent_type,
-      tools: {
-        task: false,
-        call_omo_agent: false,
+  try {
+    await ctx.client.session.prompt({
+      path: { id: sessionID },
+      body: {
+        agent: args.subagent_type,
+        tools: {
+          task: false,
+          call_omo_agent: false,
+        },
+        parts: [{ type: "text", text: args.prompt }],
       },
-      parts: [{ type: "text", text: args.prompt }],
-    },
-  })
+    })
+  } catch (error) {
+    const errorMessage = error instanceof Error ? error.message : String(error)
+    log(`[call_omo_agent] Prompt error:`, errorMessage)
+    if (errorMessage.includes("agent.name") || errorMessage.includes("undefined")) {
+      return `Error: Agent "${args.subagent_type}" not found. Make sure the agent is registered in your opencode.json or provided by a plugin.\n\n<task_metadata>\nsession_id: ${sessionID}\n</task_metadata>`
+    }
+    return `Error: Failed to send prompt: ${errorMessage}\n\n<task_metadata>\nsession_id: ${sessionID}\n</task_metadata>`
+  }
 
   log(`[call_omo_agent] Prompt sent, fetching messages...`)