adding exp artdef massdriver.yaml support

[coryodaniel]

Summary

Adds experimental support for massdriver.yaml files as artifact definitions. When mass definition publish is given a file named exactly massdriver.yaml, it now parses the experimental format and builds the artifact definition payload and publishes.

Changes

CLI (pkg/definition/)

• Added build.go with Build() function that parses massdriver.yaml and constructs the $md block with UI instructions, exports, and schema
• Modified read.go to detect massdriver.yaml files and use the new build path
• Added tests for the new functionality

massdriver.yaml Format

name: artifact-name
label: Human Label
icon: https://example.com/icon.png

ui:
  connectionOrientation: environmentDefault
  environmentDefaultGroup: credentials
  instructions:
    - label: CLI Guide
      path: ./instructions/cli.md

exports:
  - downloadButtonText: Download Config
    fileFormat: yaml
    templatePath: ./exports/config.yaml.liquid
    templateLang: liquid

schema:
  $schema: http://json-schema.org/draft-07/schema
  title: Artifact Title
  type: object
  properties:
    # ... schema properties

Test plan

• go test ./pkg/definition/... passes
• make publish-platforms successfully publishes platform definitions
• Published platforms have correct instructions and exports in the UI