run: add shell mode to prompt (#28315)

Press `!` on an empty prompt to enter shell mode and run a command
through session.shell instead of sending a message

packages/opencode/test/cli/run/subagent-data.test.ts

@@ -354,7 +354,7 @@ describe("run subagent data", () => {
     expect(visible(snapshot.details["child-1"]?.commits ?? [])).toEqual([
       "› Inspect footer tabs",
       "_Thinking:_ planning next steps",
-      "# Shell\n$ git status --short",
+      "$ git status --short",
       "hello world",
     ])
     expect(snapshot.permissions).toEqual([