fix: keep approved-spec implementations on spec branch

[oz-for-oss]

Closes #450

Summary

• Make create-implementation prompts require the exact approved spec PR branch when a plan-approved spec PR is selected.
• Keep standalone implementation prompts able to use descriptive suffix branches.
• Update the implement-issue wrapper skill and add regression coverage for both branch contracts, including rejecting approved-spec metadata branch mismatches.

Validation

• PYTHONPATH=/workspace/oz-for-oss:/workspace/oz-for-oss/core python - <<'PY' ... unittest tests.test_create_workflow_apply ... PY with minimal import-time stubs for missing third-party packages.
• python -m py_compile /workspace/oz-for-oss/core/workflows/create_implementation_from_issue.py /workspace/oz-for-oss/tests/test_create_workflow_apply.py

Notes

Full dependency-backed pytest/unittest could not be run because this sandbox has no pip, pytest, or runtime packages such as PyGithub installed.