[plan] Fix engine field duplication in included_file_schema.json

[github-actions]

Objective

Eliminate the engine field duplication risk by changing the included schema to use $ref instead of an inline definition.

Context

Priority: CRITICAL
Category: Schema Structure / Maintenance Burden

Currently:

• Main Schema: Uses $ref: "#/$defs/engine_config" (centralized definition)
• Included Schema: Uses inline definition with identical oneOf structure

This creates a maintenance burden where every engine schema change must be manually duplicated across two files, with risk of schema drift.

Approach

1. Open pkg/parser/schemas/included_file_schema.json
2. Locate the engine field definition (currently inline with oneOf structure)
3. Replace the inline definition with: "$ref": "#/$defs/engine_config"
4. Ensure the $defs/engine_config definition exists in the included schema (if not, copy from main schema)
5. Rebuild the binary with make build (schemas are embedded via //go:embed)
6. Test with a workflow that uses included files with engine configurations

Files to Modify

• Update: pkg/parser/schemas/included_file_schema.json
  • Change engine field from inline to $ref
  • Ensure $defs/engine_config is present

Acceptance Criteria

• Included schema uses $ref: "#/$defs/engine_config" for engine field
• No inline duplication of engine config structure
• Schema validates correctly with test workflows
• Binary rebuilds successfully with make build
• Included files with engine configs compile correctly

Testing

# Rebuild binary after schema changes
make build

# Test compilation with included file
./gh-aw compile test-workflow-with-includes.md

---

AI generated by Planning Assistant
Related to #7944

AI generated by Plan Command for discussion #7935