Fix repo-maintenance root resolution in generated toolkit

[gaelic-ghost]

Summary

• fix the socket-owned repo-maintenance toolkit assets in productivity-skills
• mirror the same root-resolution fix in the apple-dev-skills subtree payload
• add regression tests that run generated validate-all.sh inside a real temp git repo

Verification

• uv run python -m unittest plugins/productivity-skills/skills/maintain-project-repo/tests/test_maintain_project_repo_workflow.py
• uv run python -m unittest plugins/apple-dev-skills/tests/test_swift_package_guidance_sync_workflow.py

Closes #10