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/hooks/runtime-fallback/constants.ts

@@ -27,6 +27,7 @@ export const RETRYABLE_ERROR_PATTERNS = [
   /too.?many.?requests/i,
   /quota.?exceeded/i,
   /quota\s+will\s+reset\s+after/i,
+  /(?:you(?:'ve|\s+have)\s+)?reached\s+your\s+usage\s+limit/i,
   /all\s+credentials\s+for\s+model/i,
   /cool(?:ing)?\s+down/i,
   /exhausted\s+your\s+capacity/i,