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/tools/events.rs

@@ -56,7 +56,12 @@ pub(crate) enum ToolEventFailure {
     Message(String),
 }
 
-pub(crate) async fn emit_exec_command_begin(ctx: ToolEventCtx<'_>, command: &[String], cwd: &Path) {
+pub(crate) async fn emit_exec_command_begin(
+    ctx: ToolEventCtx<'_>,
+    command: &[String],
+    cwd: &Path,
+    is_user_shell_command: bool,
+) {
     ctx.session
         .send_event(
             ctx.turn,
@@ -65,6 +70,7 @@ pub(crate) async fn emit_exec_command_begin(ctx: ToolEventCtx<'_>, command: &[St
                 command: command.to_vec(),
                 cwd: cwd.to_path_buf(),
                 parsed_cmd: parse_command(command),
+                is_user_shell_command,
             }),
         )
         .await;
@@ -74,6 +80,7 @@ pub(crate) enum ToolEmitter {
     Shell {
         command: Vec<String>,
         cwd: PathBuf,
+        is_user_shell_command: bool,
     },
     ApplyPatch {
         changes: HashMap<PathBuf, FileChange>,
@@ -89,8 +96,12 @@ pub(crate) enum ToolEmitter {
 }
 
 impl ToolEmitter {
-    pub fn shell(command: Vec<String>, cwd: PathBuf) -> Self {
-        Self::Shell { command, cwd }
+    pub fn shell(command: Vec<String>, cwd: PathBuf, is_user_shell_command: bool) -> Self {
+        Self::Shell {
+            command,
+            cwd,
+            is_user_shell_command,
+        }
     }
 
     pub fn apply_patch(changes: HashMap<PathBuf, FileChange>, auto_approved: bool) -> Self {
@@ -110,8 +121,15 @@ impl ToolEmitter {
 
     pub async fn emit(&self, ctx: ToolEventCtx<'_>, stage: ToolEventStage) {
         match (self, stage) {
-            (Self::Shell { command, cwd }, ToolEventStage::Begin) => {
-                emit_exec_command_begin(ctx, command, cwd.as_path()).await;
+            (
+                Self::Shell {
+                    command,
+                    cwd,
+                    is_user_shell_command,
+                },
+                ToolEventStage::Begin,
+            ) => {
+                emit_exec_command_begin(ctx, command, cwd.as_path(), *is_user_shell_command).await;
             }
             (Self::Shell { .. }, ToolEventStage::Success(output)) => {
                 emit_exec_end(
@@ -200,7 +218,7 @@ impl ToolEmitter {
                 emit_patch_end(ctx, String::new(), (*message).to_string(), false).await;
             }
             (Self::UnifiedExec { command, cwd, .. }, ToolEventStage::Begin) => {
-                emit_exec_command_begin(ctx, &[command.to_string()], cwd.as_path()).await;
+                emit_exec_command_begin(ctx, &[command.to_string()], cwd.as_path(), false).await;
             }
             (Self::UnifiedExec { .. }, ToolEventStage::Success(output)) => {
                 emit_exec_end(