refactor(agents): delete dead single-agent-only middleware

file_intent (FileIntentMiddleware) and flatten_system
(FlattenSystemMessageMiddleware) were only ever instantiated in the
single-agent chat_deepagent stack, which was removed in 14bbea085. They
have no production consumer in multi_agent_chat. Delete both modules and
their unit tests.

Also drop the vestigial KnowledgeBaseSearchMiddleware alias (= the live
KnowledgePriorityMiddleware); its tests now target the real class so the
behavior coverage is preserved. Trim the three barrel/__all__ entries and
strip the now-dead class names from comments.

surfsense_backend/app/agents/shared/context.py

@@ -50,8 +50,8 @@ class SurfSenseContextSchema:
             (cloud filesystem mode). Surfaced as ``[USER-MENTIONED]``
             entries in ``<priority_documents>`` so the agent prioritises
             walking those folders with ``ls`` / ``find_documents``.
-        file_operation_contract: One-shot file operation contract emitted
-            by ``FileIntentMiddleware`` for the upcoming turn.
+        file_operation_contract: One-shot file operation contract for the
+            upcoming turn (reserved; not currently populated).
         turn_id / request_id: Correlation IDs surfaced by the streaming
             task; populated for telemetry.