feat: antropic model added fix & kb tooling fixes

- Updated main-agent middleware to clarify that both filesystem reads/writes and knowledge-base retrieval are handled by the `knowledge_base` subagent.
- Introduced `_forward_mention_pins` function to carry `@`-mention pins into subagent state.
- Revised system prompts to reflect the new retrieval method and ensure proper citation handling.
- Removed the `search_knowledge_base` tool and its related tests, consolidating functionality under the `task` tool.
- Enhanced documentation to guide usage of the new retrieval approach and citation practices.

surfsense_backend/app/agents/chat/runtime/references/models.py

@@ -8,11 +8,11 @@ a class-level discriminator used by the renderer and scope builder.
 from __future__ import annotations
 
 from dataclasses import dataclass
-from enum import Enum
+from enum import StrEnum
 from typing import ClassVar
 
 
-class ReferenceKind(str, Enum):
+class ReferenceKind(StrEnum):
     """What the user pointed at; the value is the label shown to the model."""
 
     DOCUMENT = "document"