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/plugin.js

@@ -8,6 +8,7 @@ import usecaseMustBeWired from "./rules/usecase-must-be-wired.js";
 import componentMustHaveStory from "./rules/component-must-have-story.js";
 import componentMustHaveTest from "./rules/component-must-have-test.js";
 import atomicTierImportDirection from "./rules/atomic-tier-import-direction.js";
+import piiDeclarationMustBeComplete from "./rules/pii-declaration-must-be-complete.js";
 
 /**
  * The `@repo/core-eslint` conformance plugin. Aggregates custom rules that
@@ -35,6 +36,7 @@ const plugin = {
     "component-must-have-story": componentMustHaveStory,
     "component-must-have-test": componentMustHaveTest,
     "atomic-tier-import-direction": atomicTierImportDirection,
+    "pii-declaration-must-be-complete": piiDeclarationMustBeComplete,
   },
 };