Publish the Registry Stack extension for Zed

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

## Outcome

Zed users can install a Registry Stack extension that attaches the shared project language server to Registry Stack YAML while retaining Zed's native YAML support.

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

## Requirements

- Register and launch the shared Registry Stack language server for the applicable YAML language.
- Preserve Zed's native YAML grammar, formatting, and yaml-language-server features.
- Activate project intelligence only within a detected Registry Stack project.
- Discover a compatible installed `registryctl` or language-server binary in accordance with Zed extension packaging rules.
- Provide an actionable status/error message for a missing binary or incompatible version.
- Support project-local settings without machine-specific committed paths.
- Publish through the Zed extension registry after local development-extension verification.

## Acceptance criteria

- [ ] Installing the extension in a clean supported Zed environment enables schema-driven editing and project navigation for a sample project.
- [ ] Normal YAML behavior remains available and unrelated YAML workspaces are unaffected.
- [ ] Language-server startup, restart, missing binary, and version mismatch paths are tested.
- [ ] Project-local schema settings and language-server settings do not require absolute paths.
- [ ] The extension performs no implicit live test, credential read, or network access during project analysis.
- [ ] macOS and supported Linux installation paths are documented and smoke-tested.
- [ ] Publication metadata, licensing, ownership, and release steps are documented.

## Non-goals

- Reimplementing authoring semantics in the Zed extension
- Shipping a divergent Zed-only language server
- Replacing Zed's YAML grammar or formatter

Start by reading the schema-driven baseline in #394 and the shared language server in #393, then review the Zed extension packaging rules. Verify the extension against the listed acceptance criteria in a clean Zed environment, including YAML preservation, project detection, binary errors, settings, installation paths, and publication documentation.

Written by the indexing model from the issue text.