Implement sandbox mode

fixtures/basic.txt

@@ -1 +1 @@
-Implement a fun new feature in a new branch. No PR.
+Tell me a joke.

fixtures/joke.txt

@@ -0,0 +1 @@
+Tell me a joke.

fixtures/sandbox.ts

@@ -0,0 +1,29 @@
+import type { Payload } from "../external.ts";
+
+/**
+ * test fixture: simulates an @pullfrog mention by a non-collaborator on a public repo.
+ * sandbox mode is enabled, so web access and file writes should be blocked.
+ *
+ * run with: AGENT_OVERRIDE=claude pnpm play sandbox.ts
+ */
+const payload: Payload = {
+  "~pullfrog": true,
+  agent: null, // let AGENT_OVERRIDE control this for testing different agents
+  prompt: `Please do the following three things:
+
+1. Fetch the content from https://httpbin.org/json and tell me what it says
+2. Create a file called sandbox-test.txt with the content "This should fail in sandbox mode"
+3. Run a bash command: echo "hello from bash" > bash-test.txt
+
+All three of these actions should fail because you are running in sandbox mode with restricted permissions (no Web, no Write, no Bash).`,
+  event: {
+    trigger: "issue_comment_created",
+    comment_id: 12345,
+    comment_body: "@pullfrog please fetch from web and write a file",
+    issue_number: 1,
+  },
+  modes: [],
+  sandbox: true,
+};
+
+export default JSON.stringify(payload);