refactor(agents): colocate main_agent-only kernel into main_agent/

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.

surfsense_backend/tests/unit/agents/new_chat/test_agent_cache.py

@@ -16,7 +16,7 @@ from dataclasses import dataclass
 
 import pytest
 
-from app.agents.shared.agent_cache import (
+from app.agents.multi_agent_chat.main_agent.runtime.agent_cache_store import (
     flags_signature,
     reload_for_tests,
     stable_hash,

surfsense_backend/tests/unit/agents/new_chat/test_plugin_loader.py

@@ -6,13 +6,13 @@ from unittest.mock import MagicMock, patch
 
 from langchain.agents.middleware import AgentMiddleware
 
-from app.agents.shared.plugin_loader import (
+from app.agents.multi_agent_chat.main_agent.plugins.loader import (
     PLUGIN_ENTRY_POINT_GROUP,
     PluginContext,
     load_allowed_plugin_names_from_env,
     load_plugin_middlewares,
 )
-from app.agents.shared.plugins.year_substituter import (
+from app.agents.multi_agent_chat.main_agent.plugins.year_substituter import (
     _YearSubstituterMiddleware,
     make_middleware as year_substituter_factory,
 )
@@ -66,7 +66,7 @@ class TestPluginLoaderBasics:
 
         ep = _FakeEntryPoint("dangerous_plugin", factory)
         with patch(
-            "app.agents.shared.plugin_loader.entry_points",
+            "app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points",
             return_value=[ep],
         ):
             result = load_plugin_middlewares(
@@ -78,7 +78,7 @@ class TestPluginLoaderBasics:
     def test_loads_allowlisted_plugin(self) -> None:
         ep = _FakeEntryPoint("year_substituter", year_substituter_factory)
         with patch(
-            "app.agents.shared.plugin_loader.entry_points",
+            "app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points",
             return_value=[ep],
         ):
             result = load_plugin_middlewares(
@@ -95,7 +95,7 @@ class TestPluginLoaderIsolation:
 
         ep = _FakeEntryPoint("buggy", crashing_factory)
         with patch(
-            "app.agents.shared.plugin_loader.entry_points",
+            "app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points",
             return_value=[ep],
         ):
             result = load_plugin_middlewares(_ctx(), allowed_plugin_names={"buggy"})
@@ -107,7 +107,7 @@ class TestPluginLoaderIsolation:
 
         ep = _FakeEntryPoint("liar", bad_factory)
         with patch(
-            "app.agents.shared.plugin_loader.entry_points",
+            "app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points",
             return_value=[ep],
         ):
             result = load_plugin_middlewares(_ctx(), allowed_plugin_names={"liar"})
@@ -121,7 +121,7 @@ class TestPluginLoaderIsolation:
                 raise ImportError("cannot import")
 
         with patch(
-            "app.agents.shared.plugin_loader.entry_points",
+            "app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points",
             return_value=[_BrokenEP()],
         ):
             result = load_plugin_middlewares(_ctx(), allowed_plugin_names={"broken"})
@@ -137,7 +137,7 @@ class TestPluginLoaderIsolation:
             _FakeEntryPoint("crashing", crashing_factory),
             _FakeEntryPoint("ok", year_substituter_factory),
         ]
-        with patch("app.agents.shared.plugin_loader.entry_points", return_value=eps):
+        with patch("app.agents.multi_agent_chat.main_agent.plugins.loader.entry_points", return_value=eps):
             result = load_plugin_middlewares(
                 _ctx(), allowed_plugin_names={"crashing", "ok"}
             )

surfsense_backend/tests/unit/agents/new_chat/test_skills_backends.py

@@ -7,7 +7,7 @@ from pathlib import Path
 
 import pytest
 
-from app.agents.shared.middleware.skills_backends import (
+from app.agents.multi_agent_chat.main_agent.skills.backends import (
     SKILLS_BUILTIN_PREFIX,
     SKILLS_SPACE_PREFIX,
     BuiltinSkillsBackend,

surfsense_backend/tests/unit/agents/new_chat/test_tool_call_repair.py

@@ -5,10 +5,10 @@ from __future__ import annotations
 import pytest
 from langchain_core.messages import AIMessage
 
+from app.agents.multi_agent_chat.main_agent.tools.invalid_tool import INVALID_TOOL_NAME
 from app.agents.shared.middleware.tool_call_repair import (
     ToolCallNameRepairMiddleware,
 )
-from app.agents.shared.tools.invalid_tool import INVALID_TOOL_NAME
 
 pytestmark = pytest.mark.unit