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/core/src/tasks/user_shell.rs

@@ -0,0 +1,112 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use codex_protocol::models::ShellToolCallParams;
+use codex_protocol::user_input::UserInput;
+use tokio::sync::Mutex;
+use tokio_util::sync::CancellationToken;
+use tracing::error;
+use uuid::Uuid;
+
+use crate::codex::TurnContext;
+use crate::protocol::EventMsg;
+use crate::protocol::TaskStartedEvent;
+use crate::state::TaskKind;
+use crate::tools::context::ToolPayload;
+use crate::tools::parallel::ToolCallRuntime;
+use crate::tools::router::ToolCall;
+use crate::tools::router::ToolRouter;
+use crate::turn_diff_tracker::TurnDiffTracker;
+
+use super::SessionTask;
+use super::SessionTaskContext;
+
+const USER_SHELL_TOOL_NAME: &str = "local_shell";
+
+#[derive(Clone)]
+pub(crate) struct UserShellCommandTask {
+    command: String,
+}
+
+impl UserShellCommandTask {
+    pub(crate) fn new(command: String) -> Self {
+        Self { command }
+    }
+}
+
+#[async_trait]
+impl SessionTask for UserShellCommandTask {
+    fn kind(&self) -> TaskKind {
+        TaskKind::Regular
+    }
+
+    async fn run(
+        self: Arc<Self>,
+        session: Arc<SessionTaskContext>,
+        turn_context: Arc<TurnContext>,
+        _input: Vec<UserInput>,
+        cancellation_token: CancellationToken,
+    ) -> Option<String> {
+        let event = EventMsg::TaskStarted(TaskStartedEvent {
+            model_context_window: turn_context.client.get_model_context_window(),
+        });
+        let session = session.clone_session();
+        session.send_event(turn_context.as_ref(), event).await;
+
+        // Execute the user's script under their default shell when known; this
+        // allows commands that use shell features (pipes, &&, redirects, etc.).
+        // We do not source rc files or otherwise reformat the script.
+        let shell_invocation = match session.user_shell() {
+            crate::shell::Shell::Zsh(zsh) => vec![
+                zsh.shell_path.clone(),
+                "-lc".to_string(),
+                self.command.clone(),
+            ],
+            crate::shell::Shell::Bash(bash) => vec![
+                bash.shell_path.clone(),
+                "-lc".to_string(),
+                self.command.clone(),
+            ],
+            crate::shell::Shell::PowerShell(ps) => vec![
+                ps.exe.clone(),
+                "-NoProfile".to_string(),
+                "-Command".to_string(),
+                self.command.clone(),
+            ],
+            crate::shell::Shell::Unknown => {
+                shlex::split(&self.command).unwrap_or_else(|| vec![self.command.clone()])
+            }
+        };
+
+        let params = ShellToolCallParams {
+            command: shell_invocation,
+            workdir: None,
+            timeout_ms: None,
+            with_escalated_permissions: None,
+            justification: None,
+        };
+
+        let tool_call = ToolCall {
+            tool_name: USER_SHELL_TOOL_NAME.to_string(),
+            call_id: Uuid::new_v4().to_string(),
+            payload: ToolPayload::LocalShell { params },
+        };
+
+        let router = Arc::new(ToolRouter::from_config(&turn_context.tools_config, None));
+        let tracker = Arc::new(Mutex::new(TurnDiffTracker::new()));
+        let runtime = ToolCallRuntime::new(
+            Arc::clone(&router),
+            Arc::clone(&session),
+            Arc::clone(&turn_context),
+            Arc::clone(&tracker),
+        );
+
+        if let Err(err) = runtime
+            .handle_tool_call(tool_call, cancellation_token)
+            .await
+        {
+            error!("user shell command failed: {err:?}");
+        }
+        None
+    }
+}