Improve /goal error messages for ephemeral sessions (#23796)

## Why

When a user runs `/goal` in a temporary session, the TUI can currently
surface an internal app-server failure such as `thread/goal/get failed
in TUI`. That message is technically true, but it does not explain the
actual constraint: goals require a saved session because goal state is
persisted with the thread.

This is especially confusing when `codex doctor` reports the background
app-server as running in ephemeral mode, since that wording is easy to
conflate with ephemeral thread/session behavior.

## What changed

- Added a TUI-side formatter for thread-goal RPC failures in
`codex-rs/tui/src/app/thread_goal_actions.rs`.
- Detects app-server/core errors that indicate goals are unsupported for
an ephemeral thread/session.
- Replaces the internal RPC failure with a user-facing explanation:

```text
Goals need a saved session. This session is temporary.
Run `codex` to start a saved session, or `codex resume` / `/resume` to reopen one.
```

- Preserves the existing generic failure wording for non-ephemeral goal
errors.

## Verification

- `cargo test -p codex-tui thread_goal_error_message --lib`

I also tried `cargo test -p codex-tui`; it built successfully but the
test runner aborted in an unrelated side-thread stack overflow
(`app::tests::discard_side_thread_removes_agent_navigation_entry`),
which reproduced when run by itself.

codex-rs/tui/src/app/snapshots/codex_tui__app__thread_goal_actions__tests__thread_goal_ephemeral_error_message_renders_snapshot.snap

@@ -0,0 +1,9 @@
+---
+source: tui/src/app/thread_goal_actions.rs
+expression: terminal.backend()
+---
+
+
+■ Goals need a saved session. This session is temporary.
+Run `codex` to start a saved session, or `codex resume` / `/resume` to
+reopen one.

codex-rs/tui/src/app/thread_goal_actions.rs

@@ -11,6 +11,11 @@ use crate::goal_display::goal_usage_summary;
 use codex_app_server_protocol::ThreadGoalStatus;
 use codex_protocol::ThreadId;
 
+const EPHEMERAL_THREAD_GOAL_ERROR_MESSAGE: &str = concat!(
+    "Goals need a saved session. This session is temporary.\n",
+    "Run `codex` to start a saved session, or `codex resume` / `/resume` to reopen one.",
+);
+
 impl App {
     pub(super) async fn open_thread_goal_menu(
         &mut self,
@@ -26,7 +31,7 @@ impl App {
             Ok(response) => response,
             Err(err) => {
                 self.chat_widget
-                    .add_error_message(format!("Failed to read thread goal: {err}"));
+                    .add_error_message(thread_goal_error_message("read", &err));
                 return;
             }
         };
@@ -91,7 +96,7 @@ impl App {
             Ok(response) => response,
             Err(err) => {
                 self.chat_widget
-                    .add_error_message(format!("Failed to read thread goal: {err}"));
+                    .add_error_message(thread_goal_error_message("read", &err));
                 return;
             }
         };
@@ -125,7 +130,7 @@ impl App {
                 Ok(_) => {}
                 Err(err) => {
                     self.chat_widget
-                        .add_error_message(format!("Failed to read thread goal: {err}"));
+                        .add_error_message(thread_goal_error_message("read", &err));
                     return;
                 }
             }
@@ -140,7 +145,7 @@ impl App {
                     return;
                 }
                 self.chat_widget
-                    .add_error_message(format!("Failed to replace thread goal: {err}"));
+                    .add_error_message(thread_goal_error_message("replace", &err));
                 return;
             }
         }
@@ -170,7 +175,7 @@ impl App {
             Err(err) => {
                 let action = if replacing_goal { "replace" } else { "set" };
                 self.chat_widget
-                    .add_error_message(format!("Failed to {action} thread goal: {err}"));
+                    .add_error_message(thread_goal_error_message(action, &err));
             }
         }
     }
@@ -200,7 +205,7 @@ impl App {
             ),
             Err(err) => self
                 .chat_widget
-                .add_error_message(format!("Failed to update thread goal: {err}")),
+                .add_error_message(thread_goal_error_message("update", &err)),
         }
     }
 
@@ -228,7 +233,7 @@ impl App {
             }
             Err(err) => self
                 .chat_widget
-                .add_error_message(format!("Failed to clear thread goal: {err}")),
+                .add_error_message(thread_goal_error_message("clear", &err)),
         }
     }
 
@@ -274,3 +279,75 @@ impl App {
         );
     }
 }
+
+fn thread_goal_error_message(action: &str, err: &color_eyre::Report) -> String {
+    if is_ephemeral_thread_goal_error(err) {
+        EPHEMERAL_THREAD_GOAL_ERROR_MESSAGE.to_string()
+    } else {
+        format!("Failed to {action} thread goal: {err}")
+    }
+}
+
+fn is_ephemeral_thread_goal_error(err: &color_eyre::Report) -> bool {
+    err.chain().any(|cause| {
+        let message = cause.to_string();
+        message.contains("ephemeral thread does not support goals")
+            || message.contains("thread goals require a persisted thread; this thread is ephemeral")
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::history_cell::HistoryCell;
+    use pretty_assertions::assert_eq;
+    use ratatui::layout::Rect;
+
+    use super::*;
+
+    #[test]
+    fn thread_goal_error_message_explains_temporary_session() {
+        let err = color_eyre::eyre::eyre!(
+            "thread/goal/get failed: ephemeral thread does not support goals: thread-1"
+        )
+        .wrap_err("thread/goal/get failed in TUI");
+
+        assert_eq!(
+            thread_goal_error_message("read", &err),
+            EPHEMERAL_THREAD_GOAL_ERROR_MESSAGE
+        );
+    }
+
+    #[test]
+    fn thread_goal_ephemeral_error_message_renders_snapshot() {
+        let err = color_eyre::eyre::eyre!(
+            "thread/goal/get failed: ephemeral thread does not support goals: thread-1"
+        )
+        .wrap_err("thread/goal/get failed in TUI");
+        let cell = crate::history_cell::new_error_event(thread_goal_error_message("read", &err));
+        let width = 72;
+        let height = 6;
+        let backend = crate::test_backend::VT100Backend::new(width, height);
+        let mut terminal =
+            crate::custom_terminal::Terminal::with_options(backend).expect("terminal");
+        terminal.set_viewport_area(Rect::new(0, height - 1, width, 1));
+
+        crate::insert_history::insert_history_lines(
+            &mut terminal,
+            cell.display_lines(/*width*/ width),
+        )
+        .expect("insert history lines");
+
+        insta::assert_snapshot!(terminal.backend());
+    }
+
+    #[test]
+    fn thread_goal_error_message_preserves_generic_failure_context() {
+        let err =
+            color_eyre::eyre::eyre!("server disappeared").wrap_err("thread/goal/get failed in TUI");
+
+        assert_eq!(
+            thread_goal_error_message("read", &err),
+            "Failed to read thread goal: thread/goal/get failed in TUI"
+        );
+    }
+}