Open the agents overview from an empty composer (#41917)

## What changed

- Open the agents overview with Left from an empty, focused composer in local-daemon sessions.
- Keep the shortcut inactive while editing text, using transient input surfaces, or when Left has been explicitly remapped.
- Show the contextual shortcut in the footer and shortcuts overlay.
- Leave the configurable `open_agents` action unbound by default instead of assigning `Alt+A`.

## Testing

- Add coverage for session eligibility, input-state guards, editor and Vim remaps, event dispatch, and footer rendering.

GitOrigin-RevId: ef17db3d9d09c3a08c3f7353c5b8a02f2b61c696

codex-rs/tui/src/keymap_setup.rs

@@ -1071,6 +1071,7 @@ mod tests {
                 ))
                 .collect::<Vec<_>>(),
             vec![
+                ("Open Agents", Some("unbound"), false),
                 ("Toggle Vim Mode", Some("unbound"), false),
                 ("Previous Permission Mode", Some("unbound"), false),
                 ("Next Permission Mode", Some("unbound"), false),