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>

coverage/summary.json

@@ -1,6 +1,6 @@
 {
-  "generatedAt": "2026-05-18T18:25:58.074Z",
-  "commit": "a94e803",
+  "generatedAt": "2026-05-18T18:33:27.459Z",
+  "commit": "fa1a10c",
   "repo": {
     "statements": 96.34,
     "branches": 91.41,