fix: include error details in subagent findings [AI-assisted]

When a sub-agent fails (error, timeout, or unknown status), the main
agent now receives contextual information about what went wrong instead
of just "(no output)".

Before: "Findings: (no output)"
After: "Findings: Error encountered:\n<error message>"
  or:  "Findings: Task timed out before producing output."
  or:  "Findings: Task ended with unknown status."

This helps users understand why a background task failed without needing
to inspect raw transcripts.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agents/subagent-announce.ts

@@ -384,12 +384,24 @@ export async function runSubagentAnnounceFlow(params: {
             : "finished with unknown status";
 
     // Build instructional message for main agent
+    // Include error context in findings when no reply available
+    let findings = reply;
+    if (!findings) {
+      if (outcome?.status === "error" && outcome?.error) {
+        findings = `Error encountered:\n${outcome.error}`;
+      } else if (outcome?.status === "timeout") {
+        findings = `Task timed out before producing output.`;
+      } else if (outcome?.status === "unknown") {
+        findings = `Task ended with unknown status.`;
+      }
+    }
+
     const taskLabel = params.label || params.task || "background task";
     const triggerMessage = [
       `A background task "${taskLabel}" just ${statusLabel}.`,
       "",
       "Findings:",
-      reply || "(no output)",
+      findings || "(no output)",
       "",
       statsLine,
       "",