chore(website): lint guard against new static inline styles (#858)

Lands only now that all seven migration batches (#848-#857) are merged, so it
never sees legacy code. Flags identifier-keyed members of a style object
literal in apps/website/src; the style={{ '--x': value }} escape hatch uses
string-literal keys and passes. Excluded: the two Satori OG-image files
(inline-only by design - note the [slug] segment must be matched by wildcard,
brackets are a glob character class), the dev-only primitives route, and specs.

Verified with raw eslint: fires on an identifier-key probe, silent on a
custom-property probe, zero hits on the OG file. Baseline is 18 warnings -
exactly the documented dynamic-value sites from the batch reports - and 0
errors, so CI (which fails on errors only) stays green. Ships as 'warn'; the
escalation to 'error' is a deliberate follow-up one release later.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: blove

eslint.config.mjs

@@ -54,4 +54,34 @@ export default [
     // Override or add rules here
     rules: {},
   },
+  // Inline-style guard — apps/website migrated off static inline styles
+  // (docs/superpowers/specs/2026-08-29-inline-style-substrate-migration-design.md,
+  // batches #848–#857). Flags identifier-keyed members of a style object
+  // literal. The escape hatch for dynamic values — style={{ '--x': value }} —
+  // uses string-literal keys and passes. Genuinely dynamic identifier-keyed
+  // values (rare) get a targeted eslint-disable-next-line with a reason.
+  // Ships as 'warn' for one release, then 'error'.
+  {
+    files: ['apps/website/src/**/*.tsx'],
+    ignores: [
+      // Satori-rendered OG images: inline styles are the only mechanism there.
+      'apps/website/src/app/opengraph-image.tsx',
+      // NOTE: [slug] would be a glob character class, so match by wildcard.
+      'apps/website/src/app/blog/*/opengraph-image.tsx',
+      // Dev-only route slated for deletion.
+      'apps/website/src/app/dev/primitives/page.tsx',
+      'apps/website/src/**/*.spec.tsx',
+    ],
+    rules: {
+      'no-restricted-syntax': [
+        'warn',
+        {
+          selector:
+            'JSXAttribute[name.name="style"] > JSXExpressionContainer > ObjectExpression > Property[key.type="Identifier"]',
+          message:
+            "Static presentation belongs in src/styles/*.css (see the substrate-migration spec). For dynamic values, set a CSS custom property: style={{ '--x': value }}.",
+        },
+      ],
+    },
+  },
 ];