Add custodian integration scaffold

CLAUDE.md modular rules, AGENTS/INTENT/SCOPE/.custodian-brief, and the
state-hub bootstrap workplan (WHYNOT-WP-0001).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/rules/repo-boundary.md

@@ -0,0 +1,8 @@
+## Repo boundary
+
+This repo owns **whynot-design** only. It does not own:
+
+<!-- TODO: List what belongs in adjacent repos, e.g.:
+- SSH key management → railiance-infra/
+- State hub code     → state-hub/
+-->