ci(upstream): pin gh commands to actions repo

[code-yeongyu]

Summary

Fix the upstream-agent workflow so GitHub CLI commands always target the Actions repository instead of whatever repository gh infers after the merge agent runs.

Root Cause

The failure path used gh label create and gh issue create without an explicit repository. After the merge agent ran, gh inferred earendil-works/pi, so the conflict issue was opened in the wrong repository.

Changes

• Set GH_REPO to ${{ github.repository }} for the job.
• Add --repo "$GITHUB_REPOSITORY" to PR, label, and issue gh commands in the workflow.

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/upstream-agent-merge.yml"); puts "workflow yaml ok"'
• gh pr list --repo code-yeongyu/senpi --limit 1 --json number,url --jq "."
• gh pr view 54 --repo code-yeongyu/senpi --json url,headRefOid --jq "{url,headRefOid}"
• gh pr checks 54 --repo code-yeongyu/senpi --json name,state,bucket,link --jq "map({name,state,bucket})"
• npm run check
• PR ci(upstream): pin gh commands to actions repo #55 Check and test passed.
• Manual Upstream Agent Merge retest run: https://github.com/code-yeongyu/senpi/actions/runs/27877761218
  • The run was started with force=true and dry_run=true.
  • It reached the existing Run Codex merge + changelog agent step and stayed there for more than 16 minutes, so it was cancelled before the failure-issue step.
  • No new earendil-works/pi issue was created during the retest.