Status line: support left/right alignment and terminal-width-aware layout

Feature Request

The custom status line currently renders as a simple left-aligned block. It would be useful to support right-aligned content or a left/right split layout, similar to how vim statusline or tmux status bar works.

Use Case

I'd like to display some information on the left (model, git branch) and other information on the right (token counts, cost, context usage). Currently there's no way to position content on the right side without fragile space-padding hacks.

Proposed Approach

A few possible options:

1. Tab-separated left/right — if the script outputs a tab character (\t), content before the tab is left-aligned and content after is right-aligned
2. Expose terminal width — include terminal.width in the JSON data so scripts can calculate padding themselves
3. Multiple named sections — allow the settings to define separate commands for left and right sections:

{
  "statusLine": {
    "type": "command",
    "left": "~/.claude/statusline-left.sh",
    "right": "~/.claude/statusline-right.sh"
  }
}

Any of these would enable much richer status line layouts without overcomplicating the API.