tui: add feature-gated /plan slash command to switch to Plan mode (#10103)

## Summary
Adds a simple `/plan` slash command in the TUI that switches the active
collaboration mode to Plan mode. The command is only available when the
`collaboration_modes` feature is enabled.

## Changes
- Add `plan_mask` helper in `codex-rs/tui/src/collaboration_modes.rs`
- Add `SlashCommand::Plan` metadata in
`codex-rs/tui/src/slash_command.rs`
- Implement and hard-gate `/plan` dispatch in
`codex-rs/tui/src/chatwidget.rs`
- Hide `/plan` when collaboration modes are disabled in
`codex-rs/tui/src/bottom_pane/slash_commands.rs`
- Update command popup tests in
`codex-rs/tui/src/bottom_pane/command_popup.rs`
- Add a focused unit test for `/plan` in
`codex-rs/tui/src/chatwidget/tests.rs`

## Behavior notes
- `/plan` is now a no-op if `Feature::CollaborationModes` is disabled.
- When enabled, `/plan` switches directly to Plan mode without opening
the picker.

## Codex author
`codex resume 019c05da-d7c3-7322-ae2c-3ca38d0ef702`

codex-rs/tui/src/bottom_pane/command_popup.rs

@@ -459,7 +459,7 @@ mod tests {
     #[test]
     fn collab_command_hidden_when_collaboration_modes_disabled() {
         let mut popup = CommandPopup::new(Vec::new(), CommandPopupFlags::default());
-        popup.on_composer_text_change("/coll".to_string());
+        popup.on_composer_text_change("/".to_string());
 
         let cmds: Vec<&str> = popup
             .filtered_items()
@@ -473,6 +473,10 @@ mod tests {
             !cmds.contains(&"collab"),
             "expected '/collab' to be hidden when collaboration modes are disabled, got {cmds:?}"
         );
+        assert!(
+            !cmds.contains(&"plan"),
+            "expected '/plan' to be hidden when collaboration modes are disabled, got {cmds:?}"
+        );
     }
 
     #[test]
@@ -494,6 +498,25 @@ mod tests {
         }
     }
 
+    #[test]
+    fn plan_command_visible_when_collaboration_modes_enabled() {
+        let mut popup = CommandPopup::new(
+            Vec::new(),
+            CommandPopupFlags {
+                collaboration_modes_enabled: true,
+                connectors_enabled: false,
+                personality_command_enabled: true,
+                windows_degraded_sandbox_active: false,
+            },
+        );
+        popup.on_composer_text_change("/plan".to_string());
+
+        match popup.selected_item() {
+            Some(CommandItem::Builtin(cmd)) => assert_eq!(cmd.command(), "plan"),
+            other => panic!("expected plan to be selected for exact match, got {other:?}"),
+        }
+    }
+
     #[test]
     fn personality_command_hidden_when_disabled() {
         let mut popup = CommandPopup::new(

codex-rs/tui/src/bottom_pane/slash_commands.rs

@@ -18,7 +18,10 @@ pub(crate) fn builtins_for_input(
     built_in_slash_commands()
         .into_iter()
         .filter(|(_, cmd)| allow_elevate_sandbox || *cmd != SlashCommand::ElevateSandbox)
-        .filter(|(_, cmd)| collaboration_modes_enabled || *cmd != SlashCommand::Collab)
+        .filter(|(_, cmd)| {
+            collaboration_modes_enabled
+                || !matches!(*cmd, SlashCommand::Collab | SlashCommand::Plan)
+        })
         .filter(|(_, cmd)| connectors_enabled || *cmd != SlashCommand::Apps)
         .filter(|(_, cmd)| personality_command_enabled || *cmd != SlashCommand::Personality)
         .collect()