Add VERSION file as the single source for the extension version

Mirrors testium's src/VERSION: scripts/sync-version.js copies VERSION into
package.json#version (enforcing strict semver X.Y.Z, required by the marketplace
/ Open VSX). Wired into 'npm run package' and vscode:prepublish, so the version
is always synced before a build/publish. Bump = edit VERSION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -59,9 +59,19 @@ loaded. Open any `.tum` file; the **Testium LSP** output channel shows
 the language-server traffic when `testium.trace.server` is set to
 `messages` or `verbose`.
 
+## Versioning
+
+The extension version lives in the `VERSION` file (single source of truth, like
+testium's `src/VERSION`). It must be strict semver `X.Y.Z` — the marketplace and
+Open VSX reject `X.Y`. `npm run sync-version` (run automatically by
+`npm run package` and `vscode:prepublish`) copies it into `package.json`. To
+release, bump `VERSION` and package.
+
 ## Packaging
 
 ```sh
+npm run package                  # sync VERSION -> package.json, then vsce package
+# or, when already in sync:
 npx vsce package                 # produces testium-assist-<v>.vsix
 ```