refactor(agents): move skills/, plugins/, plugin_loader to app/agents/shared (slice 7)

- 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.

surfsense_backend/app/agents/multi_agent_chat/middleware/main_agent/plugins.py

@@ -8,7 +8,7 @@ from typing import Any
 from langchain_core.language_models import BaseChatModel
 
 from app.agents.shared.feature_flags import AgentFeatureFlags
-from app.agents.new_chat.plugin_loader import (
+from app.agents.shared.plugin_loader import (
     PluginContext,
     load_allowed_plugin_names_from_env,
     load_plugin_middlewares,