feature: Add "!cmd" user shell execution (#2471)

feature: Add "!cmd" user shell execution

This change lets users run local shell commands directly from the TUI by
prefixing their input with ! (e.g. !ls). Output is truncated to keep the
exec cell usable, and Ctrl-C cleanly
  interrupts long-running commands (e.g. !sleep 10000).

**Summary of changes**

- Route Op::RunUserShellCommand through a dedicated UserShellCommandTask
(core/src/tasks/user_shell.rs), keeping the task logic out of codex.rs.
- Reuse the existing tool router: the task constructs a ToolCall for the
local_shell tool and relies on ShellHandler, so no manual MCP tool
lookup is required.
- Emit exec lifecycle events (ExecCommandBegin/ExecCommandEnd) so the
TUI can show command metadata, live output, and exit status.

**End-to-end flow**

  **TUI handling**

1. ChatWidget::submit_user_message (TUI) intercepts messages starting
with !.
2. Non-empty commands dispatch Op::RunUserShellCommand { command };
empty commands surface a help hint.
3. No UserInput items are created, so nothing is enqueued for the model.

  **Core submission loop**
4. The submission loop routes the op to handlers::run_user_shell_command
(core/src/codex.rs).
5. A fresh TurnContext is created and Session::spawn_user_shell_command
enqueues UserShellCommandTask.

  **Task execution**
6. UserShellCommandTask::run emits TaskStartedEvent, formats the
command, and prepares a ToolCall targeting local_shell.
  7. ToolCallRuntime::handle_tool_call dispatches to ShellHandler.

  **Shell tool runtime**
8. ShellHandler::run_exec_like launches the process via the unified exec
runtime, honoring sandbox and shell policies, and emits
ExecCommandBegin/End.
9. Stdout/stderr are captured for the UI, but the task does not turn the
resulting ToolOutput into a model response.

  **Completion**
10. After ExecCommandEnd, the task finishes without an assistant
message; the session marks it complete and the exec cell displays the
final output.

  **Conversation context**

- The command and its output never enter the conversation history or the
model prompt; the flow is local-only.
  - Only exec/task events are emitted for UI rendering.

**Demo video**


https://github.com/user-attachments/assets/fcd114b0-4304-4448-a367-a04c43e0b996

codex-rs/tui/src/chatwidget.rs

@@ -120,10 +120,13 @@ use codex_file_search::FileMatch;
 use codex_protocol::plan_tool::UpdatePlanArgs;
 use strum::IntoEnumIterator;
 
+const USER_SHELL_COMMAND_HELP_TITLE: &str = "Prefix a command with ! to run it locally";
+const USER_SHELL_COMMAND_HELP_HINT: &str = "Example: !ls";
 // Track information about an in-flight exec command.
 struct RunningCommand {
     command: Vec<String>,
     parsed_cmd: Vec<ParsedCommand>,
+    is_user_shell_command: bool,
 }
 
 const RATE_LIMIT_WARNING_THRESHOLDS: [f64; 3] = [75.0, 90.0, 95.0];
@@ -771,9 +774,9 @@ impl ChatWidget {
 
     pub(crate) fn handle_exec_end_now(&mut self, ev: ExecCommandEndEvent) {
         let running = self.running_commands.remove(&ev.call_id);
-        let (command, parsed) = match running {
-            Some(rc) => (rc.command, rc.parsed_cmd),
-            None => (vec![ev.call_id.clone()], Vec::new()),
+        let (command, parsed, is_user_shell_command) = match running {
+            Some(rc) => (rc.command, rc.parsed_cmd, rc.is_user_shell_command),
+            None => (vec![ev.call_id.clone()], Vec::new(), false),
         };
 
         let needs_new = self
@@ -787,6 +790,7 @@ impl ChatWidget {
                 ev.call_id.clone(),
                 command,
                 parsed,
+                is_user_shell_command,
             )));
         }
 
@@ -865,6 +869,7 @@ impl ChatWidget {
             RunningCommand {
                 command: ev.command.clone(),
                 parsed_cmd: ev.parsed_cmd.clone(),
+                is_user_shell_command: ev.is_user_shell_command,
             },
         );
         if let Some(cell) = self
@@ -875,6 +880,7 @@ impl ChatWidget {
                 ev.call_id.clone(),
                 ev.command.clone(),
                 ev.parsed_cmd.clone(),
+                ev.is_user_shell_command,
             )
         {
             *cell = new_exec;
@@ -885,6 +891,7 @@ impl ChatWidget {
                 ev.call_id.clone(),
                 ev.command.clone(),
                 ev.parsed_cmd,
+                ev.is_user_shell_command,
             )));
         }
 
@@ -1347,6 +1354,24 @@ impl ChatWidget {
 
         let mut items: Vec<UserInput> = Vec::new();
 
+        // Special-case: "!cmd" executes a local shell command instead of sending to the model.
+        if let Some(stripped) = text.strip_prefix('!') {
+            let cmd = stripped.trim();
+            if cmd.is_empty() {
+                self.app_event_tx.send(AppEvent::InsertHistoryCell(Box::new(
+                    history_cell::new_info_event(
+                        USER_SHELL_COMMAND_HELP_TITLE.to_string(),
+                        Some(USER_SHELL_COMMAND_HELP_HINT.to_string()),
+                    ),
+                )));
+                return;
+            }
+            self.submit_op(Op::RunUserShellCommand {
+                command: cmd.to_string(),
+            });
+            return;
+        }
+
         if !text.is_empty() {
             items.push(UserInput::Text { text: text.clone() });
         }