fix: report errors to progress comment when agent fails without throwing (#376)

* fix: report errors to progress comment when agent fails without throwing

when an agent returns success: false without throwing (e.g., opencode exits
with code 0 despite provider errors), the catch block in main.ts is never
reached, leaving the progress comment stuck on "leaping into action."

two fixes:
- opencode: return success: false when 0 events processed and a provider
  error was detected (converts silent failures to explicit failures)
- main.ts: after handleAgentResult, check if the progress comment was ever
  updated — if not, report the error to the comment as a safety net

Co-authored-by: Cursor <cursoragent@cursor.com>

* address review: use result directly and force failure on unreported progress

* refactor: move safety-net logic into handleAgentResult

---------

Co-authored-by: pullfrog[bot] <226033991+pullfrog[bot]@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Anna Bocharova <robin_tail@me.com>

agents/opencode.ts

@@ -437,6 +437,15 @@ export const opencode = agent({
         };
       }
 
+      if (eventCount === 0 && lastProviderError) {
+        return {
+          success: false,
+          output: finalOutput || output,
+          error: `provider error: ${lastProviderError}`,
+          usage,
+        };
+      }
+
       return {
         success: true,
         output: finalOutput || output,