refactor(agents): unify permissions into one vertical-slice package

Per-file verification of the slice-3 candidates showed receipts/ and
date_filters.py are shared contracts (consumed by shared/state + shared
middleware + subagents), so they correctly stay put.

permissions was the real misfit: the rule *model* lived at shared/permissions.py
while its enforcement lived at shared/middleware/permissions/. Unify them into a
single self-contained subsystem:

  shared/permissions.py                 -> shared/permissions/model.py
  shared/middleware/permissions/{deny,ask,middleware}
                                        -> shared/permissions/{deny,ask,middleware}

The package __init__ re-exports the model API + build_permission_mw, so the 32
external model consumers keep importing `from ...shared.permissions import Rule`
unchanged; only the 8 internal files redirect to `.model` (cycle-safe, model
loaded before middleware).

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/checkpointed_subagent_middleware/test_parallel_self_and_middleware_gated.py

@@ -43,10 +43,10 @@ from app.agents.chat.multi_agent_chat.main_agent.middleware.checkpointed_subagen
 from app.agents.chat.multi_agent_chat.main_agent.middleware.checkpointed_subagent_middleware.task_tool import (
     build_task_tool_with_parent_config,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions.ask.request import (
+from app.agents.chat.multi_agent_chat.shared.permissions import Rule
+from app.agents.chat.multi_agent_chat.shared.permissions.ask.request import (
     request_permission_decision,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule
 from app.agents.chat.multi_agent_chat.subagents.shared.hitl.approvals.self_gated import (
     request_approval,
 )

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_lc_hitl_wire.py

@@ -16,10 +16,10 @@ from langgraph.graph import END, START, StateGraph
 from langgraph.types import Command
 from typing_extensions import TypedDict
 
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions.ask.request import (
+from app.agents.chat.multi_agent_chat.shared.permissions import Rule
+from app.agents.chat.multi_agent_chat.shared.permissions.ask.request import (
     request_permission_decision,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule
 
 
 class _State(TypedDict, total=False):

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_permission_ask_mcp_context.py

@@ -14,13 +14,14 @@ from pydantic import BaseModel
 from typing_extensions import TypedDict
 
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions import (
+from app.agents.chat.multi_agent_chat.shared.permissions import (
+    Rule,
+    Ruleset,
     build_permission_mw,
 )
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions.ask.payload import (
+from app.agents.chat.multi_agent_chat.shared.permissions.ask.payload import (
     build_permission_ask_payload,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule, Ruleset
 
 
 class _NoArgs(BaseModel):

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_subagent_owned_ruleset.py

@@ -24,10 +24,11 @@ from langgraph.types import Command
 from typing_extensions import TypedDict
 
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions import (
+from app.agents.chat.multi_agent_chat.shared.permissions import (
+    Rule,
+    Ruleset,
     build_permission_mw,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule, Ruleset
 
 
 def _kb_style_ruleset() -> Ruleset:

surfsense_backend/tests/unit/agents/multi_agent_chat/middleware/shared/permissions/test_trusted_tool_save_on_always.py

@@ -15,10 +15,11 @@ from pydantic import BaseModel
 from typing_extensions import TypedDict
 
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions import (
+from app.agents.chat.multi_agent_chat.shared.permissions import (
+    Rule,
+    Ruleset,
     build_permission_mw,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule, Ruleset
 
 
 class _NoArgs(BaseModel):

surfsense_backend/tests/unit/agents/multi_agent_chat/subagents/shared/test_subagent_builder.py

@@ -20,10 +20,10 @@ from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
 from langchain_core.outputs import ChatGeneration, ChatResult
 
 from app.agents.chat.multi_agent_chat.shared.feature_flags import AgentFeatureFlags
-from app.agents.chat.multi_agent_chat.shared.middleware.permissions.middleware.core import (
+from app.agents.chat.multi_agent_chat.shared.permissions import Rule, Ruleset, evaluate
+from app.agents.chat.multi_agent_chat.shared.permissions.middleware.core import (
     PermissionMiddleware,
 )
-from app.agents.chat.multi_agent_chat.shared.permissions import Rule, Ruleset, evaluate
 from app.agents.chat.multi_agent_chat.subagents.shared.subagent_builder import (
     pack_subagent,
 )