fix: rename syntax → spec_version in example seed (#5)

Follows decree#118 which renamed the top-level seed/schema/config YAML
key from `syntax:` to `spec_version:`. Without this, `decree seed
examples/seed.yaml` against a decree built from main fails at parse.

Generated stubs (schema_service_pb2.pyi) still reference the old name
in a doc comment — will regenerate on the next decree release bump.

Co-authored-by: zeevdr <zeev.dreifuss@ownera.io>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

examples/seed.yaml

@@ -1,4 +1,4 @@
-syntax: v1
+spec_version: v1
 schema:
   name: example-app
   description: Example application configuration schema