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.

.claude/rules/repo-boundary.md

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