- skills/ (builtin SKILL.md assets) has zero Python importers; it is read by
filesystem path only. Moved the dir and restored
skills_backends._default_builtin_root() to the clean
parent.parent / "skills" / "builtin" form (undoing the transient path from 5c).
- plugin_loader.py -> shared (frozen chat_deepagent uses it -> re-export shim).
- plugins/ package -> shared (year_substituter rewired to shared.plugin_loader;
docstring entry-point example updated to the shared dotted path). No shim
needed (only a test imported it). Plugin discovery is via importlib entry
points (group "surfsense.plugins"), not dotted-path import, and nothing is
registered in pyproject, so the move does not affect runtime discovery.
