Add thread/shellCommand to app server API surface (#14988)

This PR adds a new `thread/shellCommand` app server API so clients can
implement `!` shell commands. These commands are executed within the
sandbox, and the command text and output are visible to the model.

The internal implementation mirrors the current TUI `!` behavior.
- persist shell command execution as `CommandExecution` thread items,
including source and formatted output metadata
- bridge live and replayed app-server command execution events back into
the existing `tui_app_server` exec rendering path

This PR also wires `tui_app_server` to submit `!` commands through the
new API.

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -341,6 +341,7 @@ impl ThreadHistoryBuilder {
             command,
             cwd: payload.cwd.clone(),
             process_id: payload.process_id.clone(),
+            source: payload.source.into(),
             status: CommandExecutionStatus::InProgress,
             command_actions,
             aggregated_output: None,
@@ -371,6 +372,7 @@ impl ThreadHistoryBuilder {
             command,
             cwd: payload.cwd.clone(),
             process_id: payload.process_id.clone(),
+            source: payload.source.into(),
             status,
             command_actions,
             aggregated_output,
@@ -1144,6 +1146,7 @@ impl From<&PendingTurn> for Turn {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::protocol::v2::CommandExecutionSource;
     use codex_protocol::ThreadId;
     use codex_protocol::dynamic_tools::DynamicToolCallOutputContentItem as CoreDynamicToolCallOutputContentItem;
     use codex_protocol::items::TurnItem as CoreTurnItem;
@@ -1745,6 +1748,7 @@ mod tests {
                 command: "echo 'hello world'".into(),
                 cwd: PathBuf::from("/tmp"),
                 process_id: Some("pid-1".into()),
+                source: CommandExecutionSource::Agent,
                 status: CommandExecutionStatus::Completed,
                 command_actions: vec![CommandAction::Unknown {
                     command: "echo hello world".into(),
@@ -1893,6 +1897,7 @@ mod tests {
                 command: "ls".into(),
                 cwd: PathBuf::from("/tmp"),
                 process_id: Some("pid-2".into()),
+                source: CommandExecutionSource::Agent,
                 status: CommandExecutionStatus::Declined,
                 command_actions: vec![CommandAction::Unknown {
                     command: "ls".into(),
@@ -1987,6 +1992,7 @@ mod tests {
                 command: "echo done".into(),
                 cwd: PathBuf::from("/tmp"),
                 process_id: Some("pid-42".into()),
+                source: CommandExecutionSource::Agent,
                 status: CommandExecutionStatus::Completed,
                 command_actions: vec![CommandAction::Unknown {
                     command: "echo done".into(),