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>

CLAUDE.md

@@ -72,7 +72,7 @@ Every feature has a `src/feature.manifest.ts` declaring its use cases, audits, p
 | **CI drift gate** (`pnpm conformance`) | ~120s   | orphan event consumers across features                                                                      |
 | **Fallow** (`pnpm fallow`)             | ~30–60s | dead exports / unused files; duplicate code; circular deps; complexity hotspots; AI-change audit drift      |
 
-The seven conformance ESLint rules: `feature-must-have-manifest` (error), `usecase-must-have-test-file` (error), `required-cores-installed` (error), `usecase-must-be-wired` (error), `no-undeclared-event-publish` (warn), `no-undeclared-audit` (warn), `no-undeclared-analytics-event` (warn). Fallow runs as a fifth layer, post-ESLint, whole-codebase.
+The eight conformance ESLint rules: `feature-must-have-manifest` (error), `usecase-must-have-test-file` (error), `required-cores-installed` (error), `usecase-must-be-wired` (error), `no-undeclared-event-publish` (warn), `no-undeclared-audit` (warn), `no-undeclared-analytics-event` (warn), `pii-declaration-must-be-complete` (warn). Fallow runs as a fifth layer, post-ESLint, whole-codebase.
 
 See `docs/architecture/agent-first-workflow-and-conformance.md` for the full design and `docs/guides/conformance-quickref.md` for the day-to-day reference.