Add /compact (#1527)

- Add operation to summarize the context so far.
- The operation runs a compact task that summarizes the context.
- The operation clear the previous context to free the context window
- The operation didn't use `run_task` to avoid corrupting the session
- Add /compact in the tui



https://github.com/user-attachments/assets/e06c24e5-dcfb-4806-934a-564d425a919c

codex-rs/tui/src/slash_command.rs

@@ -13,6 +13,7 @@ pub enum SlashCommand {
     // DO NOT ALPHA-SORT! Enum order is presentation order in the popup, so
     // more frequently used commands should be listed first.
     New,
+    Compact,
     Diff,
     Quit,
     #[cfg(debug_assertions)]
@@ -24,6 +25,7 @@ impl SlashCommand {
     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)"