Publish the Registry Stack extension for VS Code

Tracked by #399. Depends on #394 and #393.

## Outcome

VS Code users can install a Registry Stack extension that combines the existing schema-driven YAML experience with the shared project language server and common `registryctl` project actions.

Depends on the schema-driven baseline and shared Registry Stack language server.

## Requirements

- Activate only when a workspace contains or opens a Registry Stack project.
- Integrate with the Red Hat YAML extension for normal YAML validation, completion, hover, outline, and formatting.
- Launch and supervise the shared Registry Stack language server.
- Discover a compatible `registryctl` installation or configured language-server binary.
- Report missing or incompatible binaries with an actionable installation/configuration message.
- Expose project commands for:
- `registryctl check`;
- offline `registryctl test`;
- `registryctl build`.
- Never start live tests or read credentials implicitly.
- Display command output through standard VS Code progress/output/problem surfaces.
- Package for the Visual Studio Marketplace and Open VSX where policy and ownership allow.

## Acceptance criteria

- [ ] Installing the extension in a clean supported VS Code environment enables the schema and language-server features for a sample project.
- [ ] The extension does not claim unrelated YAML files.
- [ ] Language-server startup, restart, crash, missing binary, and version mismatch paths are tested.
- [ ] Check, offline test, and build require an explicit user command and use the selected project/environment.
- [ ] No command logs secret values or full environment contents.
- [ ] Extension packaging is reproducible and has automated smoke tests on supported platforms.
- [ ] Installation, binary discovery, settings, and troubleshooting are documented.
- [ ] A release process and ownership location are documented before marketplace publication.

## Non-goals

- Implementing project semantics in TypeScript
- Bundling a second copy of the Registry Stack compiler
- Automatically running live integration tests

Start by reviewing the schema-driven baseline and shared Registry Stack language server in #394 and #393, then identify the VS Code extension entry points and packaging configuration. Validate activation, binary discovery, language-server failure paths, explicit check/offline test/build commands, and smoke tests in a clean supported environment. Done includes reproducible marketplace packaging and documentation for installation, settings, troubleshooting, and release ownership.

Written by the indexing model from the issue text.