feat: move environment option into build stanza and document it
The environment cog.yaml option is a build-time setting that adds
environment variables to the Docker image, so it belongs in the build
stanza rather than at the top level.
- Move Environment field from Config to Build struct
- Accept top-level environment with a deprecation warning for backward
compatibility; error if set in both places
- Add environment to the JSON schema under build properties
- Add documentation to docs/yaml.md
- Update integration test and YAML overwrite test fixtures
- Regenerate docs/llms.txt
