Remappable keyboard navigation for worktree sidebar and terminal panes

### Problem or use case

A keyboard-first Orca workflow still requires mouse interaction or several disconnected commands when moving among the worktree sidebar and terminal panes.

The desired flow is:

1. Press one remappable shortcut, for example **Alt+D**, to reveal the left sidebar and focus the worktree list.
2. Use **j** and **k** to move through worktrees while that list is focused.
3. Use remappable directional shortcuts matching an existing tmux setup to move directly between terminal panes.

Today, focusing the worktree list and toggling sidebar visibility are separate concepts, list navigation is not available through scoped Vim-style keys, and pane navigation does not provide a complete set of remappable directional actions suitable for tmux-style bindings.

### Proposed solution

Add first-class, remappable commands for keyboard-only navigation:

- `sidebar.toggleAndFocusWorktreeList`
- If the sidebar is hidden, show it and focus the worktree list.
- If it is visible but focus is elsewhere, focus the worktree list.
- If the worktree list is already focused, toggle the sidebar closed and return focus to the active terminal/editor.
- While the worktree list itself is focused, support remappable next/previous actions with **j**/**k** as possible bindings. These keys must remain scoped to the list and must not intercept text entry in search fields, editors, or terminals.
- Add directional pane actions such as `terminal.focusPaneLeft`, `terminal.focusPaneRight`, `terminal.focusPaneUp`, and `terminal.focusPaneDown` so users can reproduce their tmux navigation bindings.
- Expose all actions in Settings -> Shortcuts and handle conflicts consistently with the existing keybinding registry.

The exact defaults are less important than making the actions independently remappable. **Alt+D**, **j**/**k**, and the users existing tmux chord can then be configured without patching Orca.

### Alternatives or additional context

Related but distinct requests:

- #6674 requests shortcuts for switching between projects.
- #9939 covered restoring terminal focus after switching worktrees.
- #8584 covered a conflicting default binding for focusing the worktree list.

This request is specifically for one-step sidebar toggle/focus, focus-scoped worktree-list navigation, and directional terminal-pane bindings.