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/codex_app_server_protocol.v2.schemas.json

@@ -1026,6 +1026,30 @@
           "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": {
@@ -2754,6 +2778,15 @@
       "title": "CommandExecutionOutputDeltaNotification",
       "type": "object"
     },
+    "CommandExecutionSource": {
+      "enum": [
+        "agent",
+        "userShell",
+        "unifiedExecStartup",
+        "unifiedExecInteraction"
+      ],
+      "type": "string"
+    },
     "CommandExecutionStatus": {
       "enum": [
         "inProgress",
@@ -9823,6 +9856,14 @@
                 "null"
               ]
             },
+            "source": {
+              "allOf": [
+                {
+                  "$ref": "#/definitions/CommandExecutionSource"
+                }
+              ],
+              "default": "agent"
+            },
             "status": {
               "$ref": "#/definitions/CommandExecutionStatus"
             },
@@ -10862,6 +10903,29 @@
       "title": "ThreadSetNameResponse",
       "type": "object"
     },
+    "ThreadShellCommandParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "command": {
+          "description": "Shell command string evaluated by the thread's configured shell. Unlike `command/exec`, this intentionally preserves shell syntax such as pipes, redirects, and quoting. This runs unsandboxed with full access rather than inheriting the thread sandbox policy.",
+          "type": "string"
+        },
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "command",
+        "threadId"
+      ],
+      "title": "ThreadShellCommandParams",
+      "type": "object"
+    },
+    "ThreadShellCommandResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "title": "ThreadShellCommandResponse",
+      "type": "object"
+    },
     "ThreadSortKey": {
       "enum": [
         "created_at",