[plan] Document safe-outputs scope restrictions in reference docs

[github-actions]

Objective

Add documentation explaining why safe-outputs has limited scope in included files compared to main workflows.

Context

The analysis found an intentional design restriction:

• Main workflows support 38 safe-output operation types (issues, discussions, comments, labels, pull-requests, agent-tasks, etc.)
• Included files only support 1 operation type (jobs)
• The schema description states: "only jobs allowed in included files"

This restriction is not documented in the reference documentation, making it unclear why included files have reduced capabilities.

Approach

1. Review docs/src/content/docs/reference/ structure
2. Locate or create appropriate documentation file for safe-outputs
3. Add section explaining:
   • Main workflows can use all safe-output types
   • Included files are restricted to jobs only
   • Rationale for the restriction
   • Examples of each use case
4. Reference the schema descriptions as source of truth

Files to Modify

• Create or update file in docs/src/content/docs/reference/ directory
• Possibly update docs/src/content/docs/reference/safe-outputs.md if it exists

Acceptance Criteria

• Documentation clearly states safe-outputs scope difference
• Explanation provided for why included files have limited scope
• Examples show correct usage in both main and included files
• Documentation linked from relevant guides or overview pages
  Related to [plan] Fix schema consistency issues found in analysis #8804

AI generated by Plan Command for discussion #8801