feat: /fork the current session instead of opening session picker (#9385)

Implemented /fork to fork the current session directly (no picker),
handling it via a new ForkCurrentSession app event in both tui and tui2.
Updated slash command descriptions/tooltips and adjusted the fork tests
accordingly. Removed the unused in-session fork picker event.

codex-rs/tui/src/slash_command.rs

@@ -48,7 +48,7 @@ impl SlashCommand {
             SlashCommand::Compact => "summarize conversation to prevent hitting the context limit",
             SlashCommand::Review => "review my current changes and find issues",
             SlashCommand::Resume => "resume a saved chat",
-            SlashCommand::Fork => "fork a saved chat",
+            SlashCommand::Fork => "fork the current chat",
             // SlashCommand::Undo => "ask Codex to undo a turn",
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
             SlashCommand::Diff => "show git diff (including untracked files)",