feat(grid-agent): enforce central action policy (#120)

crates/metacrate-grid-agent/README.md

@@ -40,4 +40,7 @@ cargo run --locked -p metacrate-grid-agent -- \
 See [`../../docs/grid-agent-architecture.md`](../../docs/grid-agent-architecture.md)
 for queue/task ownership, shutdown, and trust boundaries. See
 [`../../docs/grid-agent-llm.md`](../../docs/grid-agent-llm.md) for the LLM wire
-compatibility envelope, retry rules, and tool-loop safety contract.
+compatibility envelope, retry rules, and tool-loop safety contract. The central
+origin matrix, approval binding, budgets, prompt-data boundary, and opaque
+backend authorization are specified in
+[`../../docs/grid-agent-policy.md`](../../docs/grid-agent-policy.md).