Restart conversation support.

The idea here is to let the user restart the conversation, possibly with
a better prompt. This is a common manual workflow, and I'd like to make
it easier.

I hand wrote the agent.go stuff, but Sketch wrote the rest.

Co-Authored-By: sketch <hello@sketch.dev>
diff --git a/webui/src/web-components/sketch-container-status.test.ts b/webui/src/web-components/sketch-container-status.test.ts
index db43a6d..f8dc685 100644
--- a/webui/src/web-components/sketch-container-status.test.ts
+++ b/webui/src/web-components/sketch-container-status.test.ts
@@ -24,6 +24,8 @@
   outstanding_tool_calls: [],
   session_id: "test-session-id",
   ssh_available: false,
+  in_container: true,
+  first_message_index: 0,
 };
 
 test("render props", async ({ mount }) => {