docs(impeccable): prohibit inner max-width caps; override 65-75ch rule

Expands the layout constraint to make explicit that --shell-max on
.section-shell is the ONLY width cap allowed. Blocks future additions
of narrow inner caps on cards, table cells, paragraphs, etc., and
explicitly disables the impeccable skill's default 65-75ch guidance.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vinta

.impeccable.md

@@ -65,7 +65,10 @@ The project already follows these. Future work must keep them.
 
 Layout and sizing:
 
-- Keep existing `--shell-max: 84rem` (~1344px). Do not drop below 1280px for the main column.
+- Keep existing `--shell-max: 84rem` (~1344px) applied via `.section-shell`. This is the ONLY width cap in the project. Widescreen monitors are the default viewing context.
+- Do NOT add `max-width` to sections, cards, table cells, table rows, expanded rows, CTA backgrounds, sponsor descriptions, hero subcopy, paragraphs, or list items. The user has removed narrow inner caps repeatedly (`56ch`, `65-75ch`, etc.). Default is no inner cap.
+- The impeccable skill reference rule "cap line length at ~65-75ch" does NOT apply here. Ignore it. Readability at wide widths is carried by vertical rhythm, leading, and the modular type scale instead.
+- If you believe a width cap is actually necessary for some specific element, ask first with a concrete reason before adding it.
 - Body type floor is 16px (`--text-base: 1rem`). Content-heavy passages may go to 1.125rem.
 - Adjacent heading levels differ by at least 0.25rem of rendered size.