add slash resume (#7302)

`codex resume` isn't that discoverable. Adding it to the slash commands
can help

codex-rs/tui/src/snapshots/codex_tui__resume_picker__tests__resume_picker_screen.snap

@@ -0,0 +1,14 @@
+---
+source: tui/src/resume_picker.rs
+assertion_line: 1438
+expression: snapshot
+---
+Resume a previous session
+Type to search
+  Updated  Branch  CWD  Conversation
+No sessions yet
+
+
+
+
+enter to resume     esc to start new     ctrl + c to quit