agent: retire eager KB priority/planner path and its dead flags

The pull-based KB design (on-demand search_knowledge_base tool + pre-injected
workspace tree) fully replaced the old eager retrieval path. Remove its last
remnants:

- Delete KnowledgePriorityMiddleware (knowledge_search.py) and its tests.
- Drop the kb_priority state field + reducer default; trim
  KbContextProjectionMiddleware to project only workspace_tree_text.
- Remove the now-dead feature flags enable_kb_priority_preinjection and
  enable_kb_planner_runnable across backend (flags, route schema, tests,
  env examples) and frontend (settings toggle, zod schema).
- Scrub <priority_documents> and stale KnowledgePriorityMiddleware references
  from prompts, docstrings, and the ADR.

No functional change: nothing wrote kb_priority and neither flag gated live
behavior after the cutover. Full backend suite green (pre-existing unrelated
failures aside).

surfsense_backend/app/tasks/chat/streaming/flows/new_chat/runtime_context.py

@@ -22,7 +22,8 @@ def build_new_chat_runtime_context(
     request_id: str | None,
     turn_id: str,
 ) -> SurfSenseContextSchema:
-    """``mentioned_document_ids`` is consumed by ``KnowledgePriorityMiddleware``.
+    """``mentioned_document_ids`` is consumed by the ``search_knowledge_base``
+    tool (via ``referenced_document_ids``) to pin mentioned docs into scope.
 
     ``accepted_folder_ids`` (post-resolve) wins over the raw
     ``mentioned_folder_ids`` from the request: the resolver drops chips that