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/v2/ThreadStartResponse.json

@@ -353,6 +353,15 @@
         }
       ]
     },
+    "CommandExecutionSource": {
+      "enum": [
+        "agent",
+        "userShell",
+        "unifiedExecStartup",
+        "unifiedExecInteraction"
+      ],
+      "type": "string"
+    },
     "CommandExecutionStatus": {
       "enum": [
         "inProgress",
@@ -1236,6 +1245,14 @@
                 "null"
               ]
             },
+            "source": {
+              "allOf": [
+                {
+                  "$ref": "#/definitions/CommandExecutionSource"
+                }
+              ],
+              "default": "agent"
+            },
             "status": {
               "$ref": "#/definitions/CommandExecutionStatus"
             },