[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**
```json
[]
```

Start with the /statusline documentation and the statusline-setup agent referenced in the request. Verify how the rendered width relates to COLUMNS and inspect how generated right-aligned scripts calculate width, including codepoint and terminal glyph-width concerns. Done means the usable-width rule is documented and generated scripts account for the reserve and stated width caveats.

Written by the indexing model from the issue text.