feat(core-eslint): add entity-must-have-test and no-relative-parent-import rules

Two CLAUDE.md conventions had no mechanical gate, so both drifted:
entity models shipped without sibling tests, and feature test files
imported src modules via `../` instead of the `@/` alias.

- `entity-must-have-test` — every entities/models/<x>.ts needs a sibling
  <x>.test.ts (errors and barrels excluded).
- `no-relative-parent-import-in-tests` — feature test files must import
  src via `@/`, not `../`. Scoped to feature packages; core packages are
  governed by their own generator templates.

Both register at warn level, bringing the conformance rule count to 15.

packages/core-eslint/rules/entity-must-have-test.js

@@ -0,0 +1,55 @@
+import fs from "node:fs";
+import path from "node:path";
+
+/**
+ * Entity models (`entities/models/<x>.ts`) are pure domain logic — schemas,
+ * invariants, derivations. They are the cheapest layer to test and the most
+ * expensive to get wrong, so every model file must carry a sibling test.
+ *
+ * Scope is `entities/models/` only. Error classes (`entities/errors/`) are
+ * conventionally covered by a consolidated `errors.test.ts`, and barrels
+ * (`index.ts`) hold no logic — both are excluded.
+ */
+function isEntityModelFile(filename) {
+  const normalized = filename.replace(/\\/g, "/");
+  if (!normalized.includes("/entities/models/")) return false;
+  if (!normalized.endsWith(".ts")) return false;
+  if (normalized.endsWith(".test.ts")) return false;
+  if (normalized.endsWith(".d.ts")) return false;
+  if (normalized.endsWith("/index.ts")) return false;
+  return true;
+}
+
+/** @type {import("eslint").Rule.RuleModule} */
+export default {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Every entity model file (entities/models/<x>.ts) must have a sibling <x>.test.ts.",
+    },
+    schema: [],
+    messages: {
+      missingTest:
+        "Entity model {{filename}} has no sibling test at {{expected}}. Entity models are pure domain logic — cover them with a unit test.",
+    },
+  },
+  create(context) {
+    return {
+      Program(node) {
+        const filename = context.filename;
+        if (!isEntityModelFile(filename)) return;
+        const expected = filename.replace(/\.ts$/, ".test.ts");
+        if (fs.existsSync(expected)) return;
+        context.report({
+          node,
+          messageId: "missingTest",
+          data: {
+            filename: path.basename(filename),
+            expected: path.basename(expected),
+          },
+        });
+      },
+    };
+  },
+};