Add shared slash-command status preamble guidance

[Copilot]

Slash-command workflows repeatedly duplicate the same status-comment: true and reaction UX pattern. This adds a shared documentation import to centralize the recommended preamble without forcing workflow-specific trigger fields into a shared frontmatter fragment.

• Shared guidance

  • Added .github/workflows/shared/slash-command-status-preamble.md
  • Documents the expected on.slash_command + status-comment + reaction pattern
  • Captures centralized vs decentralized strategy guidance

• Import-ready example

imports:
- shared/slash-command-status-preamble.mdon:
slash_command:
name: mycmdstrategy: centralizedreaction: eyesstatus-comment: true

• Fixes[refactoring] Skill extraction: slash-command status-comment/reaction preamble guidance #50760