#286 Docs: document self.* expressions in modelIdOrName during forEach

Problem Statement

The workflow reference docs don't document that self.* expressions now resolve in modelIdOrName and methodName during forEach expansion.

Proposed Solution

Update content/manual/reference/workflows.md to document this capability in the forEach section, with an example like:

steps:
  - name: summary-${{ self.region }}
    forEach:
      item: region
      in: ${{ inputs.regions }}
    task:
      type: model_method
      modelIdOrName: aws-alarms-${{ self.region }}
      methodName: get_summary
      inputs:
        historyHours: 24

This enables forEach steps to target different model instances per iteration without requiring a nested workflow-of-workflows pattern.

Alternatives Considered

N/A — straightforward documentation update.