feat(#1942): VS Code extension — repo-local IDE host with reachability proof (#1966)

* feat(#1942): VS Code extension — repo-local IDE host with reachability proof

The VS Code extension (vscode/extension.js + vscode/package.json) is a
repo-local, buildable extension (not Marketplace-published). The gsd.invoke
handler dispatches in-process through the GSD command-routing hub. The handler
is exported separately from activate() so it is testable without a VS Code host.

tests/vscode-extension-reachability.test.cjs: proves the handler dispatches
through the hub + returns a result (keystone wired); the manifest declares the
command + engine; resolveEngineRoot finds the gsd-core/ dir.

* docs(changeset): VS Code extension (#1966)

* fix(#1942): register vscode/package.json in VERSIONED_MANIFESTS (#844)

* fix(#1942): align vscode/package.json version with package.json (1.7.0-rc.1)

scripts/sync-manifest-versions.cjs

@@ -34,6 +34,7 @@ const VERSIONED_MANIFESTS = [
   { path: '.claude-plugin/plugin.json', versionKey: 'version' },
   { path: 'gemini-extension.json', versionKey: 'version' },
   { path: '.claude-plugin/marketplace.json', versionKey: 'plugins.0.version' },
+  { path: 'vscode/package.json', versionKey: 'version' },
 ];
 
 // Convenience: just the registered paths, for consumers that only need to