Add codex-grid-background detector rule (#328)

* Add codex-grid-background detector rule

Detects the Codex two-axis grid-line background tell: a single background
value carrying two or more hairline `linear-gradient(... 1px, transparent
1px)` layers (one per axis), usually paired with a repeating
`background-size` cell. Gated behind --gpt like the sibling codex tells,
off by default.

Counts hairline stops within a single background declaration (not across
the page) so unrelated single-axis ruled lines don't add up to a false
flag, and matches the stop directly rather than parsing whole gradient
layers, since colors like oklch(...) carry nested parens.

Extends the gpt-tells fixture with one flag case and two pass cases
(single-axis rule, two-color blend), regenerates the browser detector
bundle, and bumps the rule count 44 -> 45.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Require tiling background-size for codex-grid-background

Address review: two hairline gradients alone draw a fixed crosshair, not a
grid. Scope detection to a single style block (CSS rule body or inline
style attr) and require both >=2 hairline stops AND a tiling
`background-size` px cell in the same block, matching the skill rule's
"plus background-size" wording. Add a crosshair-without-tiling pass case.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Scope codex-grid-background hairline count to background values

Address review: count hairline stops only inside background/background-image
declaration values, not the whole style block, so a hairline in an unrelated
property (mask-image, border-image) can't stand in for the grid's second
axis. Add a bg+mask-image hairline pass case.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cli/engine/registry/antipatterns.mjs

@@ -376,6 +376,17 @@ const ANTIPATTERNS = [
     skillSection: 'Visual Details',
     skillGuideline: 'repeating-gradient decorative stripes',
   },
+  {
+    id: 'codex-grid-background',
+    category: 'slop',
+    severity: 'advisory',
+    gated: 'gpt',
+    name: 'Decorative grid-line background',
+    description:
+      'A two-axis grid drawn with hairline linear-gradient layers ("1px, transparent 1px" on both axes) is a recurring generated-UI signature. Reserve grid overlays for actual canvas, map, blueprint, or measurement surfaces; elsewhere use product structure or a plain surface.',
+    skillSection: 'Visual Details',
+    skillGuideline: 'two-axis grid-line gradient background',
+  },
   {
     id: 'theater-slop-phrase',
     category: 'slop',