ci(sync): pin the hotspot issue to the fork repo

[64ix]

Description

Third and last piece of the upstream-sync repair. The sync's ff+push now works (verified by dispatch), but the final step — opening the Upstream touched fork hotspots issue — failed with GraphQL: Resource not accessible by integration (createIssue).

Cause: gh run inside a fork checkout resolves to the parent repository by default. The step was trying to create the issue on generalaction/emdash with a token scoped to 64ix/emdash. Same failure class as the PR generalaction#2976 incident in AGENTS.md, this time for issues.

Fix: set GH_REPO: ${{ github.repository }} on the step so every gh call targets the fork.

Related issues

Follow-up to #164 (sync push auth) and #163 (superseded).

Testing

• Dispatched the workflow after merging ci(sync): push the main mirror with an optional PAT instead of an ungrantable workflows scope #164: ff+push succeeded, only the issue step failed — with this pin it targets the fork where the token has issues: write
• No hotspot issue ever existed before (gh issue list --state all empty), confirming this step had never successfully run

Checklist

• I kept this PR small and focused
• I ran a self-review before opening this PR
• I ran the relevant local checks or explained why not
• I updated docs when behavior or setup changed
• I added or updated tests when behavior changed, or explained why not
• I only added comments where the logic is not obvious
• I used Conventional Commits for commit messages and, when possible, the PR title