Define Rust environment and toolchain

AGENTS.md

@@ -4,7 +4,7 @@
 
 These instructions apply to the entire repository. Keep documentation, code comments, and identifiers in English; use the user's language in conversation.
 
-The project currently contains design/configuration documents, not an implemented Rust application. Update this statement when scaffolding is created. Do not report planned code, tests, or commands as already working.
+The project contains a Rust package with startup, settings validation, and a pure client domain model. The management interface and VPN operations are not implemented yet. Do not report planned modules, tests, or commands as already working.
 
 - Read [SPEC.md](docs/SPEC.md) and [CONTEXT.md](CONTEXT.md) before implementing behavior. The approved specification is the product contract; do not change it to make tests pass.
 - For milestone execution, read and maintain [PLANS.md](PLANS.md), following [IMPLEMENTATION.md](docs/IMPLEMENTATION.md). Execute numbered milestones sequentially through implementation, independent review, repair, and acceptance with evidence.
@@ -26,6 +26,8 @@ The project currently contains design/configuration documents, not an implemente
 
 ## Development rules and reference map
 
+Use the pinned Rust toolchain and environment checks in [ENVIRONMENT.md](docs/ENVIRONMENT.md). System-packaged tools do not automatically honor rust-toolchain.toml.
+
 | When working on                                           | Read first                                  |
 | --------------------------------------------------------- | ------------------------------------------- |
 | Module layout and boundaries                              | [PROJECT_LAYOUT.md](docs/PROJECT_LAYOUT.md) |