feat(plan): add pre-flight check for duplicate upstream PRs and Issues

[Ty-Robb]

Summary

• Added a new gstack-check-duplicates command that uses the GitHub CLI to list recent open Pull Requests and Issues on the upstream repository.
• Integrated the command into the PRE-REVIEW SYSTEM AUDIT phase of both the CEO and Engineering plan review skills.
• Agents now automatically check for overlapping work before writing a new spec.
• Gracefully handles cases where the GitHub CLI is not installed or not authenticated.

Test plan

• Verified the command lists open PRs and Issues when the GitHub CLI is authenticated.
• Verified the command prints a helpful fallback message when the GitHub CLI is not installed.
• Verified the command prints a helpful fallback message when not authenticated.
  🤖 Authored via Spec-Driven Development