fix(bin): read cross-repo PR checks from the head repository

[mehulbhagwani]

Problem

fm-contributions.sh queried check-runs and commit statuses against the
PR's base repository, even when the PR's head commit only exists on a
fork. GitHub's commits API silently returns zero results for a SHA it
cannot resolve in that repository, so a contribution whose PR page is
on the base repo but whose commits live on a fork (e.g. a PR opened
mehulbhagwani:branch -> kunchenguid/firstmate:main) was observed as
having no checks at all instead of its real check-run results.

Reproduced directly against #4932:
querying repos/kunchenguid/firstmate/commits/<sha>/check-runs returns
total_count: 0, while repos/mehulbhagwani/firstmate/commits/<sha>/check-runs
returns the real 20 check-runs for that commit.

Fix

Read .head.repo.full_name from the PR core response and query
check-runs and statuses there instead of the base repo, falling back to
the base repository when the head repo field is absent (a deleted
fork).

Testing

• Reproduced against the real PR via a scratch FM_HOME before and
  after the fix, confirming checks went from empty to the real 20
  check-runs with no error.
• Added test_cross_repo_checks_use_head_repo to
  tests/fm-contributions.test.sh, asserting check-runs/statuses are
  read from the head repo and not the base repo when they differ.
• tests/fm-contributions.test.sh passes (40/40).

[mehulbhagwani]

Superseded by #5919; will close once it lands.

[mehulbhagwani]

Closing as superseded by #5919, which carries the same fix rebased on current main.