Fix /goal usage text for control commands (#26551)

## Why

The TUI's `/goal` usage text only advertised the objective form even
though `/goal clear`, `/goal edit`, `/goal pause`, and `/goal resume`
are implemented. This made the lifecycle controls difficult to discover
and allowed the duplicated help text to drift from actual behavior.

Fixes #25530.

## What changed

- Show the complete `/goal [<objective>|clear|edit|pause|resume]` syntax
in usage messages.
- Share one usage string across slash-command dispatch and goal-related
app messages.
- Add inline snapshot coverage for the control-command usage path.

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -13,6 +13,7 @@ use crate::bottom_pane::slash_commands::BuiltinCommandFlags;
 use crate::bottom_pane::slash_commands::ServiceTierCommand;
 use crate::bottom_pane::slash_commands::SlashCommandItem;
 use crate::bottom_pane::slash_commands::find_slash_command;
+use crate::goal_display::GOAL_USAGE;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 enum SlashCommandDispatchSource {
@@ -32,7 +33,6 @@ struct PreparedSlashCommandArgs {
 const SIDE_STARTING_CONTEXT_LABEL: &str = "Side starting...";
 const SIDE_SLASH_COMMAND_UNAVAILABLE_HINT: &str =
     "Press Ctrl+C to return to the main thread first.";
-const GOAL_USAGE: &str = "Usage: /goal <objective>";
 const GOAL_USAGE_HINT: &str = "Example: /goal improve benchmark coverage";
 const RAW_USAGE: &str = "Usage: /raw [on|off]";