docs: add beta notice and STABILITY.md

Add beta banner to README and create STABILITY.md linking to the
canonical component status table in dev-toolchain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: matthew-on-git

README.md

@@ -1,5 +1,7 @@
 # devrail.dev
 
+> **Beta** -- DevRail is under active development. Standards, tooling, and site content may change without notice. See [STABILITY.md](STABILITY.md) for details.
+
 The documentation site for [DevRail](https://devrail.dev) -- opinionated development standards for teams that ship with AI agents.
 
 ## Quick Start

STABILITY.md

@@ -0,0 +1,25 @@
+# Stability
+
+DevRail is in **beta** (`v0.x`). The project is under active development.
+
+## What "Beta" Means
+
+- **Standards may change.** Section content, naming, and structure may be revised as we learn from real-world usage.
+- **Tooling may change.** Tool versions, container contents, and Makefile targets may be updated without prior deprecation.
+- **No backward-compatibility guarantee.** Until `v1.0.0`, any release may include breaking changes. Pin to a specific tag if you need stability.
+- **Feedback is welcome.** File issues, open discussions, or submit PRs. Early adopters shape the final product.
+
+## Component Status
+
+For the full component-by-component status table, see the [dev-toolchain STABILITY.md](https://github.com/devrail-dev/dev-toolchain/blob/main/STABILITY.md).
+
+## Versioning
+
+All DevRail repos use `v0.x.y` during beta. The `0.x` range signals that breaking changes may occur in any release, per [Semantic Versioning](https://semver.org/) conventions.
+
+Once the core standards, toolchain image, and CI templates are stable and validated in production use, we will release `v1.0.0` and commit to backward compatibility.
+
+## How to Track Changes
+
+- Watch the [CHANGELOG.md](CHANGELOG.md) in each repo for release-by-release details.
+- Breaking changes will be called out explicitly in changelog entries during beta.