refactor(source-control): babysit scripts read the tracker binding from the convention home #11881

Problem

The babysit merge, policy, skip and post-merge scripts read .work-item-tracker.json directly.
ADR 0075 fix 11 asks for a named YAML reader for Python.

Origin: Q551; ADR 0075 fix 11.

Fix

One Python reader for <home>/work-items.yaml (with the JSON fallback while K19's fallback lasts),
used by every babysit script that reads the binding, reading YAML the way babysit_merge_policy.py
already does (yaml-subset.awk records, no PyYAML); the safety reference names it. The merge gate
reads the binding from the default branch today, and so does the new reader; babysit_merge_skip.py
MAIN_PATHS keys on the new file as it does on the JSON one.

Done when

• git grep -l -F '.work-item-tracker' -- plugins/source-control ':!**/CHANGELOG.md' lists only the reader and its test.
• The babysit tests pass under scripts/run-test-file.sh, including a new reader test for YAML, fallback and absent.
• A source-control changelog fragment passes bash scripts/check-changelog-fragments.sh.

Parent: #7578