fix: implement pending_messages cleanup to prevent unbounded growth

Fixes #353 - Observations not being saved due to incomplete pending messages implementation

Changes:
- PendingMessageStore.markProcessed() now clears tool_input and tool_response
- PendingMessageStore.cleanupProcessed() changed from time-based to count-based retention
- Keeps most recent 100 processed messages for UI display
- SDKAgent.processSDKResponse() calls cleanup after marking messages processed

This prevents the database from growing unbounded with duplicate transcript data.
The pending_messages table now only stores full transcripts for pending/processing
messages, while processed messages keep metadata only.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/services/worker/SDKAgent.ts

@@ -409,6 +409,14 @@ export class SDKAgent {
         count: session.pendingProcessingIds.size
       });
       session.pendingProcessingIds.clear();
+
+      // Clean up old processed messages (keep last 100 for UI display)
+      const deletedCount = pendingMessageStore.cleanupProcessed(100);
+      if (deletedCount > 0) {
+        logger.debug('SDK', 'Cleaned up old processed messages', {
+          deletedCount
+        });
+      }
     }
 
     // Broadcast activity status after processing (queue may have changed)