[plan] Update compiler to support custom action references

[github-actions]

Objective

Modify the workflow compiler to generate references to custom actions instead of embedding JavaScript inline via actions/github-script.

Context

The compiler currently uses DefaultScriptRegistry and bundling to embed JavaScript. We need to add a mode where it emits uses: references to custom actions instead.

Approach

1. Add a compilation mode flag (e.g., --action-mode=custom|inline)
2. Update pkg/workflow/compiler.go to support custom action references
3. Modify step generation to emit:

   # Development mode
   - uses: ./actions/check-membership
   
   # Release mode (to be implemented in separate task)
   - uses: githubnext/gh-aw/.github/actions/check-membership@SHA

4. Update script_registry.go to support mapping script names to action paths
5. Add tests for both inline and custom action modes

Files to Modify

• pkg/workflow/compiler.go
• pkg/workflow/script_registry.go
• pkg/workflow/bundler.go (if needed)
• Add tests: pkg/workflow/compiler_custom_actions_test.go

Acceptance Criteria

• Compiler can generate workflows using custom action references
• Development mode uses local paths (./actions/...)
• Action inputs are properly mapped from script parameters
• All existing inline tests still pass
• New tests validate custom action mode compilation
• No breaking changes to existing workflows
  Related to [plan] Refactor compiler to use custom actions for JavaScript code #5947

AI generated by Plan Command for #5946