[Feature Request] Document statusline usable width and fix right-aligned element truncation

Bug Description
The /statusline docs imply a script can use the full COLUMNS width, but in practice Claude Code renders the status-line row in a field ~3 columns narrower than COLUMNS (a fixed right margin), and truncates any overflow with a …. A right-anchored element (e.g. a context-usage bar) built to COLUMNS therefore loses its last characters to …, with no way to discover the reserve from the docs.

Requests:

1. Document the exact usable width (e.g. "the status line renders in COLUMNS − N; build to that, not COLUMNS").
2. Have the statusline-setup agent account for it when generating right-aligned scripts.
3. Related gotchas worth documenting/handling in generated scripts: width math must count codepoints, not bytes (multibyte Nerd Font glyphs need a UTF-8 locale), and Nerd Font glyph cell-width varies by font/terminal (1 vs 2 cells), which shifts alignment.

Environment Info

• Platform: linux
• Terminal: WezTerm
• Version: 2.1.197
• Feedback ID: b54eaf06-e663-42cc-864a-d646237a2a7c

Errors

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