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/exec/src/event_processor_with_human_output.rs

@@ -278,6 +278,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 command,
                 cwd,
                 parsed_cmd: _,
+                is_user_shell_command: _,
             }) => {
                 self.call_id_to_command.insert(
                     call_id,