feature: Add "!cmd" user shell execution

- protocol: add Op::RunUserShellCommand to model a user-initiated one-off command
- core: handle new Op by spawning a cancellable task that runs the command using the user’s default shell; stream output via ExecCommand* events; send TaskStarted/TaskComplete; track as current_task so Interrupt works
- tui: detect leading '!' in composer submission and dispatch Op::RunUserShellCommand instead of sending a user message

No changes to sandbox env var behavior; uses existing exec pipeline and event types.

codex-rs/protocol/src/protocol.rs

@@ -176,6 +176,16 @@ pub enum Op {
 
     /// Request to shut down codex instance.
     Shutdown,
+
+    /// Execute a user-initiated one-off shell command (triggered by "!cmd").
+    ///
+    /// The command string is executed using the user's default shell and may
+    /// include shell syntax (pipes, redirects, etc.). Output is streamed via
+    /// `ExecCommand*` events and the UI regains control upon `TaskComplete`.
+    RunUserShellCommand {
+        /// The raw command string after '!'
+        command: String,
+    },
 }
 
 /// Determines the conditions under which the user is consulted to approve
@@ -1052,6 +1062,10 @@ pub struct ExecCommandBeginEvent {
     /// The command's working directory if not the default cwd for the agent.
     pub cwd: PathBuf,
     pub parsed_cmd: Vec<ParsedCommand>,
+    /// True when this exec was initiated directly by the user (e.g. bang command),
+    /// not by the agent/model. Defaults to false for backwards compatibility.
+    #[serde(default)]
+    pub is_user_shell_command: bool,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, TS)]