Add directional conversation focus shortcuts in workspace mode (#59287)

## What changed

- Add configurable `focus_pane_left` and `focus_pane_right` actions, defaulting to `leader left` / `leader h` and `leader right` / `leader l`.
- Move focus to the parent or open side conversation without reopening a hidden pane. Preserve the current interaction when no neighbor is available, and accept fresh typing after a no-op shortcut while suppressing reported key repeats.
- Expose the actions in the keymap picker and configuration schema, and prevent workspace-only pane bindings from claiming keys in classic mode.

## Testing

Add regression coverage for pane navigation, preserved transcript selection and search, fresh typing after a no-op, key capture, and binding conflicts and limits.

GitOrigin-RevId: 7f7629d15cffcd35d1353b36c5c9208dbb1034e8

codex-rs/config/src/tui_keymap.rs

@@ -143,6 +143,10 @@ pub struct TuiGlobalKeymap {
     pub focus_sidebar_content: Option<KeybindingsSpec>,
     /// Hide the focused sidebar pane.
     pub hide_pane: Option<KeybindingsSpec>,
+    /// Focus the parent conversation.
+    pub focus_pane_left: Option<KeybindingsSpec>,
+    /// Focus the open side conversation.
+    pub focus_pane_right: Option<KeybindingsSpec>,
     /// Open the shared agent-session overview.
     pub open_agents: Option<KeybindingsSpec>,
     /// Open the transcript overlay.