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/error-classifier.test.ts

@@ -253,6 +253,17 @@ describe("quota error detection (fixes #2747)", () => {
     expect(retryable).toBe(true)
   })
 
+  test("treats hard usage-limit wording as retryable", () => {
+    //#given
+    const error = { message: "You've reached your usage limit for this month. Please upgrade to continue." }
+
+    //#when
+    const retryable = isRetryableError(error, [429, 503])
+
+    //#then
+    expect(retryable).toBe(true)
+  })
+
   test("classifies QuotaExceededError by errorName even without quota keywords in message", () => {
     //#given
     const error = { name: "QuotaExceededError", message: "Request failed." }