Move environment option into build stanza and document it

[bfirsh]

Summary

• Moves the environment cog.yaml option from the top level into the build: stanza, since it's a build-time setting that adds env vars to the Docker image
• Adds documentation for environment in docs/yaml.md
• Maintains backward compatibility: top-level environment still works but emits a deprecation warning

Changes

Config structs: Environment and parsedEnvironment moved from Config to Build. The canonical location is now build.environment.

Backward compatibility: A new resolveEnvironment() validation step handles migration:

• Top-level only → migrated into build.environment with a deprecation warning
• build.environment only → no-op
• Both set → validation error

JSON schema: environment added to build properties; top-level kept with deprecation note.

Documentation: New ### environment section under ## build in docs/yaml.md (alphabetically between cuda and gpu). Documents the NAME=value format, provides an example, and notes reserved variable names.

Tests: Integration test and YAML overwrite test fixtures updated to use the new location.

Example

# New (preferred)
build:
  python_version: "3.12"
  environment:
    - NAME=michael
    - TEST_VAR=test_value

# Old (still works, emits deprecation warning)
build:
  python_version: "3.12"
environment:
  - NAME=michael
  - TEST_VAR=test_value