feat: wire fork to codex cli (#8994)

## Summary
- add `codex fork` subcommand and `/fork` slash command mirroring resume
- extend session picker to support fork/resume actions with dynamic
labels in tui/tui2
- wire fork selection flow through tui bootstraps and add fork-related
tests

codex-rs/tui2/src/chatwidget.rs

@@ -1547,6 +1547,9 @@ impl ChatWidget {
             SlashCommand::Resume => {
                 self.app_event_tx.send(AppEvent::OpenResumePicker);
             }
+            SlashCommand::Fork => {
+                self.app_event_tx.send(AppEvent::OpenForkPicker);
+            }
             SlashCommand::Init => {
                 let init_target = self.config.cwd.join(DEFAULT_PROJECT_DOC_FILENAME);
                 if init_target.exists() {