fix(ci): add jsonschema to [dev] extras

test_bundled_schema_matches_model in tests/core/test_config.py
imports jsonschema to validate the shipped release-kit.schema.json
against Config.example(). The dep was implicit on my local venv
(pulled in by an unrelated install) but missing from pyproject's
[dev] block, so a clean CI install of `pip install -e .[dev]`
hit ModuleNotFoundError on macOS / Linux / Windows × py3.11/12/13.

Floor pinned at 4.21 because draft-2020-12 validator stabilised
there.

Author: imanimanyara

pyproject.toml

@@ -70,6 +70,7 @@ dev = [
   "mypy>=1.10",
   "pre-commit>=3.7",
   "types-PyYAML",
+  "jsonschema>=4.21",   # validates the bundled release-kit.schema.json
 ]
 
 [project.scripts]