Move modules out of agents/shared/ that are consumed by a single package
(main_agent), placing each next to its only consumer instead of in a
"shared" grab-bag:
- agent_cache.py -> main_agent/runtime/agent_cache_store.py
- connector_searchable_types.py -> main_agent/runtime/
- plugin_loader.py + plugins/ -> main_agent/plugins/
- skills/ + skills_backends.py -> main_agent/skills/
- tools/invalid_tool.py -> main_agent/tools/
Drop the skills_backends re-export from the shared middleware barrel and
repoint all consumers + tests. No behavior change; import-all,
error-contract, and the moved tests stay green.