docs: charter meta-framework vision, research, and SAND-WP-0002

Rewrite INTENT.md as the sand-boxer meta-framework charter (OpenRouter-style
sandbox API, extensions, payments, Coulomb sibling boundaries). Add research
under research/, update SCOPE.md, bootstrap workplans SAND-WP-0001/0002, and
State Hub integration files from the bootstrap pass.

.custodian-brief.md

@@ -0,0 +1,31 @@
+<!-- custodian-brief: generated by statehub register; fix-consistency may replace this file -->
+# Custodian Brief - sand-boxer
+
+**Project:** sand-boxer
+**Domain:** infotech
+**State Hub:** http://127.0.0.1:8000
+**Topic ID:** `cee7bedf-2b48-46ef-8601-006474f2ad7a`
+
+## Open Workplans
+
+### Bootstrap State Hub integration
+
+Workplan file: `workplans/SAND-WP-0001-statehub-bootstrap.md` (status: active)
+
+Open tasks:
+- T02 - Verify local developer workflow
+
+### Meta-framework foundation and first extension
+
+Workplan file: `workplans/SAND-WP-0002-meta-framework-foundation.md` (status: ready)
+
+Next: T01 - Design meta-framework contracts
+
+## Session Start
+
+1. Read `INTENT.md`, `SCOPE.md`, and `AGENTS.md`.
+2. Check inbox: `GET /messages/?to_agent=sand-boxer&unread_only=true`.
+3. Scan `workplans/`.
+4. Update task statuses in workplan files as work progresses.
+
+Last generated: 2026-06-22