feat(tui) /clear (#12444)

# /clear feature! 

/clear will clear your terminal while preserving the context/state of
the thread.

codex-rs/tui/src/slash_command.rs

@@ -47,6 +47,7 @@ pub enum SlashCommand {
     Rollout,
     Ps,
     Clean,
+    Clear,
     Personality,
     TestApproval,
     // Debugging commands.
@@ -67,6 +68,7 @@ impl SlashCommand {
             SlashCommand::Review => "review my current changes and find issues",
             SlashCommand::Rename => "rename the current thread",
             SlashCommand::Resume => "resume a saved chat",
+            SlashCommand::Clear => "clear the terminal screen and scrollback",
             SlashCommand::Fork => "fork the current chat",
             // SlashCommand::Undo => "ask Codex to undo a turn",
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
@@ -136,6 +138,7 @@ impl SlashCommand {
             | SlashCommand::Experimental
             | SlashCommand::Review
             | SlashCommand::Plan
+            | SlashCommand::Clear
             | SlashCommand::Logout
             | SlashCommand::MemoryDrop
             | SlashCommand::MemoryUpdate => false,