chore(template): clean-slate template snapshot from bb4a0c7

Curated, product-agnostic snapshot of the post-story-04 tree: demo
content deleted, auth-only reference feature, web-next shell, all gates
green. Product-specific docs, ADRs 027-029, PRDs/epics/archive, editor
library traces, and product naming are curated out; generic template
repairs (coverage provider devDeps, root test:coverage script, live
lint fixes, root-only release-please) are kept. See TEMPLATE.md for
provenance, curation list, and usage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-eslint/rules/usecase-must-have-test-file.js

@@ -0,0 +1,38 @@
+import fs from "node:fs";
+
+/** @type {import("eslint").Rule.RuleModule} */
+export default {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Every *.use-case.ts file must have a sibling *.use-case.test.ts (TDD discipline).",
+    },
+    schema: [],
+    messages: {
+      missingTestFile:
+        "Use case {{filename}} has no sibling test file at {{expected}}. Write the red test first.",
+    },
+  },
+  create(context) {
+    return {
+      Program(node) {
+        const filename = context.filename;
+        if (!filename.endsWith(".use-case.ts")) return;
+        const expected = filename.replace(
+          /\.use-case\.ts$/,
+          ".use-case.test.ts",
+        );
+        if (fs.existsSync(expected)) return;
+        context.report({
+          node,
+          messageId: "missingTestFile",
+          data: {
+            filename: filename.split("/").pop(),
+            expected: expected.split("/").pop(),
+          },
+        });
+      },
+    };
+  },
+};