0.4.0: feat(templates): optional templates + a plugin-release template (#13)

Optional templates, and a plugin-release template for single-source versioning and automated releases.

What changed

Trellis can now ship optional templates - opt-in bundles a repo installs with
/trellis-install --template <name>, kept current thereafter by plain /trellis-update. Unlike
rules (which every Trellis repo follows), a template applies only to repos that want it.

The first template is plugin-release, for repos that are themselves published marketplace
plugins:

• A root VERSION file is the single source of truth; scripts/bump-version.sh rewrites it and
  every manifest listed in .version-manifests together, and --check fails CI on any drift.
• A standalone .github/workflows/release.yml tags the version and publishes a GitHub Release
  once your CI workflow succeeds on main - composing with the existing "What's new" automation.

Each template splits into owned/ (Trellis refreshes these on update) and seed/ (copied once,
then yours), so an update can never clobber your content. See trellis/templates/README.md.

Dogfood

Trellis adopts plugin-release for its own seven manifests (previously hand-synced at 0.3.0),
adds a CI workflow that runs its test suites and the version-sync guard, and cuts this 0.4.0
release through the new pattern.