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.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "bun:test"
+import { transformMcpServer } from "./transformer"
+
+describe("transformMcpServer", () => {
+  describe("#given a remote MCP server with oauth config", () => {
+    it("#when transforming the server #then preserves oauth on the remote config", () => {
+      const transformed = transformMcpServer("remote-oauth", {
+        type: "http",
+        url: "https://mcp.example.com",
+        headers: { Authorization: "Bearer test" },
+        oauth: {
+          clientId: "client-id",
+          scopes: ["read", "write"],
+        },
+      })
+
+      expect(transformed).toEqual({
+        type: "remote",
+        url: "https://mcp.example.com",
+        headers: { Authorization: "Bearer test" },
+        oauth: {
+          clientId: "client-id",
+          scopes: ["read", "write"],
+        },
+        enabled: true,
+      })
+    })
+  })
+})