Workflows currently have no way to declare pricing for custom or private models, nor to override built-in cost entries. This adds a models frontmatter field (using the same structure as models.json) that merges into models.json at runtime before AWF receives it.

Approach

Mirrors the existing model_multipliers.json merge pattern exactly:

1. Compiler serializes the models overlay to JSON → emits as GH_AW_INFO_MODEL_COSTS in the activation job
2. generate_aw_info.cjs merges it over the built-in models.json → writes /tmp/gh-aw/models.json
3. Activation artifact includes /tmp/gh-aw/models.json; agent job receives it via GH_AW_MODELS_JSON_PATH

Merge semantics: overlay wins at model granularity (whole model entry replaced); unmentioned providers/models preserved from base.

The models field is disambiguated at parse time: when the value contains a top-level providers key it is treated as a models.json overlay; otherwise it falls through to the deprecated alias-map format for backward compatibility.

Changes

• Go (frontmatter_types.go, frontmatter_parsing.go, compiler_types.go, workflow_builder.go, compiler_yaml.go) — parse and emit the new field; ParseFrontmatterConfig detects the providers-keyed structure and routes it to ModelCosts
• awf_helpers.go — GH_AW_MODELS_JSON_PATH now points to /tmp/gh-aw/models.json (the merged file) instead of the setup-copied original
• compiler_activation_job_builder.go — include /tmp/gh-aw/models.json in activation artifact upload
• merge_frontmatter_models.cjs (new) — mergeModelCosts() + writeMergedModelsJSON(); called from generate_aw_info.cjs
• main_workflow_schema.json — models property updated to oneOf: providers structure (new) or deprecated alias-array format (backward compat)

Usage

models:
  providers:
    anthropic:
      models:
        my-custom-claude:
          cost:
            input: "3e-06"
            output: "1.5e-05"
            cache_read: "3e-07"
            cache_write: "3.75e-06"