Add focus container-next / container-prev for sibling-only cycling

[Knapsack44]

I'd like to add two focus commands:

• focus container-next
• focus container-prev

Motivation

dfs-next / dfs-prev are workspace-wide. For accordion-style layouts, I want to cycle only between sibling windows inside the currently focused window's parent container.

Behavior

• Start from the focused window.
• Move only within its direct parent container.
• Cycle among that container's child windows.
• Wrap around.
• No-op if there is only one focusable child.
• No-op or fail consistently if there is no suitable parent container.

Example

ctrl-tab = 'focus container-next'
ctrl-shift-tab = 'focus container-prev'

Visual example

Workspace
├── Left pane: Window 1
└── Accordion container
    ├── Window 2
    └── Window 3

focus container-next:
  Window 2 -> Window 3 -> Window 2

focus container-prev:
  Window 2 -> Window 3 -> Window 2

Important:
- cycling stays inside the accordion container
- it does not jump to Window 1

Why this approach

• Explicit and minimal.
• Does not change existing DFS behavior.
• Fits the "focus sibling only" use case without shell hacks or simulated key presses.

This proposal and the corresponding implementation were prepared by Codex, supervised by me. A draft PR with the required changes already exists and is ready for review.