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.md

@@ -0,0 +1,12 @@
+# whynot-design — Claude Code Instructions
+
+@SCOPE.md
+@.claude/rules/repo-identity.md
+@.claude/rules/session-protocol.md
+@.claude/rules/first-session.md
+@.claude/rules/workplan-convention.md
+@.claude/rules/stack-and-commands.md
+@.claude/rules/architecture.md
+@.claude/rules/repo-boundary.md
+@.claude/rules/credential-routing.md
+@.claude/rules/agents.md