docs: add missing safe-outputs fields to github-agentic-workflows.md (#20435)

Add documentation for two safe-output global config fields that exist in
the code and JSON schema but were missing from the instructions file:

- report-failure-as-issue: controls whether workflow failures create issues
- safe-outputs.environment: overrides deployment environment for the
  safe-outputs job (separate from the top-level environment: field)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]