fix: resolve 3 community-reported bugs (#2915, #2917, #2918)

- background_output: snapshot read cursor before consuming, restore on
  /undo message removal so re-reads return data (fixes #2915)
- MCP loader: preserve oauth field in transformMcpServer, add scope/
  projectPath filtering so local-scoped MCPs only load in matching
  directories (fixes #2917)
- runtime-fallback: add 'reached your usage limit' to retryable error
  patterns so quota exhaustion triggers model fallback (fixes #2918)

Verified: bun test (4606 pass / 0 fail), tsc --noEmit clean

src/features/claude-code-mcp-loader/transformer.ts

@@ -30,6 +30,10 @@ export function transformMcpServer(
       config.headers = expanded.headers
     }
 
+    if (expanded.oauth && Object.keys(expanded.oauth).length > 0) {
+      config.oauth = expanded.oauth
+    }
+
     return config
   }