feat(core-eslint): add pii-declaration-must-be-complete rule

Adds the `conformance/pii-declaration-must-be-complete` ESLint rule at
warn severity. The rule detects `custom: { pii: { ... } }` blocks in
Payload config files and warns when any of the four required sub-fields
(`category`, `purpose`, `exportable`, `restrictable`) is missing.

Incomplete PII declarations can produce incorrect audit reports —
sub-second editor feedback catches the gap before it reaches
compliance/data-map.yml.

- Rule + 7 RuleTester fixtures (complete passes, each missing field
  warns, non-pii custom block is no-op, malformed custom.pii is no-op)
- Registered in plugin.js + base.js at "warn"
- Conformance rule count bumped 7 → 8 in CLAUDE.md +
  conformance-quickref.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-eslint/rules/pii-declaration-must-be-complete.js

@@ -0,0 +1,57 @@
+const REQUIRED_FIELDS = ["category", "purpose", "exportable", "restrictable"];
+
+/** @type {import("eslint").Rule.RuleModule} */
+export default {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "custom.pii blocks in Payload config files must declare all required sub-fields: category, purpose, exportable, restrictable.",
+    },
+    schema: [],
+    messages: {
+      missingField:
+        "custom.pii block is missing required field '{{field}}'. Incomplete PII declarations can produce incorrect audit reports.",
+    },
+  },
+  create(context) {
+    return {
+      Property(node) {
+        if (
+          node.key.type !== "Identifier" ||
+          node.key.name !== "custom" ||
+          node.value.type !== "ObjectExpression"
+        ) {
+          return;
+        }
+
+        const piiProp = node.value.properties.find(
+          (p) =>
+            p.type === "Property" &&
+            p.key.type === "Identifier" &&
+            p.key.name === "pii",
+        );
+
+        if (!piiProp || piiProp.value.type !== "ObjectExpression") {
+          return;
+        }
+
+        const presentFields = new Set(
+          piiProp.value.properties
+            .filter((p) => p.type === "Property" && p.key.type === "Identifier")
+            .map((p) => p.key.name),
+        );
+
+        for (const field of REQUIRED_FIELDS) {
+          if (!presentFields.has(field)) {
+            context.report({
+              node: piiProp,
+              messageId: "missingField",
+              data: { field },
+            });
+          }
+        }
+      },
+    };
+  },
+};