statusLine config: support position field to align custom status to the right side of the row

Feature Request

Currently the custom statusLine always renders on the left side of its dedicated row. The right side is hardcoded for Claude Code's own system notifications. There is no way to control horizontal alignment.

Requested Change

Add a position field to the statusLine config in settings.json:

"statusLine": {
  "type": "command",
  "command": "...",
  "padding": 0,
  "position": "right"
}

Accepted values: "left" (default, current behaviour) | "right" | "center"

Use Case

The bottom row is visually busy. Placing the custom statusLine on the right — mirrored to where system notifications appear — would make it feel like a persistent indicator rather than leftover text. This is the convention in most developer tools: primary status on the left, contextual metrics (tokens, cost, git) on the right.

Examples of tools that follow this pattern:

• tmux — status-right / status-left are separate config fields
• vim/neovim statusline — right-aligned sections via %= separator
• VS Code status bar — items have explicit alignment: Left | Right

Current Workaround

Padding with spaces is the only option, but it breaks across different terminal widths and is not reliable.

Impact

Low-risk change — it only affects rendering position of user-supplied output, not any core functionality.