Make steer stable by default (openai#10690)

Promotes the Steer feature from Experimental to Stable and enables it by
default.

## What is Steer mode?

Steer mode changes how message submission works in the TUI:

- **With Steer enabled (new default)**: 
  - `Enter` submits messages immediately, even when a task is running
- `Tab` queues messages when a task is running (allows building up a
queue)
  
- **With Steer disabled (old behavior)**:
  - `Enter` queues messages when a task is running
  - This preserves the previous "queue while a task is running" behavior

## How Steer vs Queue work

The key difference is in the submission behavior:

1. **Steer mode** (`steer_enabled = true`):
- Enter → `InputResult::Submitted` → sends immediately via
`submit_user_message()`
- Tab → `InputResult::Queued` → queues via `queue_user_message()` if a
task is running
- This gives users direct control: Enter for immediate submission, Tab
for queuing

2. **Queue mode** (`steer_enabled = false`, previous default):
- Enter → `InputResult::Queued` → always queues when a task is running
   - Tab → `InputResult::Queued` → queues when a task is running
- This preserves the original behavior where Enter respects the running
task queue

## Implementation details

The behavior is controlled in
`ChatComposer::handle_key_event_without_popup()`:
- When `steer_enabled` is true, Enter calls `handle_submission(false)`
(submit immediately)
- When `steer_enabled` is false, Enter calls `handle_submission(true)`
(queue)

See `codex-rs/tui/src/bottom_pane/chat_composer.rs` for the
implementation.

## Documentation

For more details on the chat composer behavior, see:
- [TUI Chat Composer documentation](docs/tui-chat-composer.md)
- Feature flag definition: `codex-rs/core/src/features.rs`

Author: aibrahim-oai

codex-rs/core/src/features.rs

@@ -556,12 +556,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::Steer,
         key: "steer",
-        stage: Stage::Experimental {
-            name: "Steer conversation",
-            menu_description: "Enter submits immediately; Tab queues messages when a task is running.",
-            announcement: "NEW! Try Steer mode: Enter submits immediately, Tab queues. Enable in /experimental!",
-        },
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::CollaborationModes,