Add thread/shellCommand to app server API surface (#14988)

This PR adds a new `thread/shellCommand` app server API so clients can
implement `!` shell commands. These commands are executed within the
sandbox, and the command text and output are visible to the model.

The internal implementation mirrors the current TUI `!` behavior.
- persist shell command execution as `CommandExecution` thread items,
including source and formatted output metadata
- bridge live and replayed app-server command execution events back into
the existing `tui_app_server` exec rendering path

This PR also wires `tui_app_server` to submit `!` commands through the
new API.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -2881,6 +2881,22 @@
       ],
       "type": "object"
     },
+    "ThreadShellCommandParams": {
+      "properties": {
+        "command": {
+          "description": "Shell command string evaluated by the thread's configured shell. Unlike `command/exec`, this intentionally preserves shell syntax such as pipes, redirects, and quoting. This runs unsandboxed with full access rather than inheriting the thread sandbox policy.",
+          "type": "string"
+        },
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "command",
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ThreadSortKey": {
       "enum": [
         "created_at",
@@ -3586,6 +3602,30 @@
       "title": "Thread/compact/startRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "thread/shellCommand"
+          ],
+          "title": "Thread/shellCommandRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadShellCommandParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Thread/shellCommandRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {