Restore legacy AWF model-multiplier shim to prevent agent startup crashes

[Copilot]

gh-aw v0.77.5 workflows could fail before Copilot execution in the agent job due to a missing runtime module: merge_awf_model_multipliers.cjs. The failure pattern indicates older compiled workflow scripts still reference that legacy filename even after multiplier handling moved to the newer frontmatter merge module.

• Backward-compatibility shim

  • Added actions/setup/js/merge_awf_model_multipliers.cjs as a compatibility entrypoint.
  • The shim delegates to merge_frontmatter_models.cjs and preserves legacy surface area, including writeMergedModelMultipliersJSON.

• Legacy API continuity

  • Re-exported current merge helpers from the shim so old require paths keep working without changing runtime behavior.
  • Kept implementation minimal to avoid introducing a second merge path.

• Regression coverage

  • Added actions/setup/js/merge_awf_model_multipliers.test.cjs to assert shim exports and legacy alias availability.

// actions/setup/js/merge_awf_model_multipliers.cjs
const frontmatterModels = require("./merge_frontmatter_models.cjs");

module.exports = {
  ...frontmatterModels,
  writeMergedModelMultipliersJSON: frontmatterModels.writeMergedModelsJSON,
};

[pelikhan]

@copilot validate that this issue is not present on the current build, do not install legacy shim.