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/architecture.md

@@ -0,0 +1,8 @@
+## Architecture
+
+<!-- TODO: Describe the key design decisions and component structure.
+     Key modules, data flows, external integrations, state machines, etc. -->
+
+## Quick Reference
+
+`~/state-hub/mcp_server/TOOLS.md` — MCP tool reference