[feat] add /status slash command (#1873)

- Added a `/status` command, which will be useful when we update the
home screen to print less status.
- Moved `create_config_summary_entries` to common since it's used in a
few places.
- Noticed we inconsistently had periods in slash command descriptions
and just removed them everywhere.
- Noticed the diff description was overflowing so made it shorter.

codex-rs/tui/src/slash_command.rs

@@ -15,6 +15,7 @@ pub enum SlashCommand {
     New,
     Compact,
     Diff,
+    Status,
     Quit,
     #[cfg(debug_assertions)]
     TestApproval,
@@ -24,12 +25,11 @@ impl SlashCommand {
     /// User-visible description shown in the popup.
     pub fn description(self) -> &'static str {
         match self {
-            SlashCommand::New => "Start a new chat.",
-            SlashCommand::Compact => "Compact the chat history.",
-            SlashCommand::Quit => "Exit the application.",
-            SlashCommand::Diff => {
-                "Show git diff of the working directory (including untracked files)"
-            }
+            SlashCommand::New => "Start a new chat",
+            SlashCommand::Compact => "Compact the chat history",
+            SlashCommand::Quit => "Exit the application",
+            SlashCommand::Diff => "Show git diff (including untracked files)",
+            SlashCommand::Status => "Show current session configuration and token usage",
             #[cfg(debug_assertions)]
             SlashCommand::TestApproval => "Test approval request",
         }