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/tests/unit/agents/new_chat/test_compaction.py

@@ -38,7 +38,7 @@ class TestIsProtectedSystemMessage:
         )
 
     def test_tolerates_leading_whitespace(self) -> None:
-        msg = SystemMessage(content="   \n<priority_documents>\n...")
+        msg = SystemMessage(content="   \n<workspace_tree>\n...")
         assert _is_protected_system_message(msg) is True
 
 
@@ -89,7 +89,7 @@ class TestPartitionMessages:
 
     def test_protected_system_message_preserved_even_in_summarize_half(self) -> None:
         partitioner = self._build_partitioner()
-        protected = SystemMessage(content="<priority_documents>\n...")
+        protected = SystemMessage(content="<workspace_tree>\n...")
         msgs = [
             HumanMessage(content="old human"),
             AIMessage(content="old ai"),

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

@@ -28,7 +28,6 @@ def _clear_all(monkeypatch: pytest.MonkeyPatch) -> None:
         "SURFSENSE_ENABLE_LLM_TOOL_SELECTOR",
         "SURFSENSE_ENABLE_SKILLS",
         "SURFSENSE_ENABLE_SPECIALIZED_SUBAGENTS",
-        "SURFSENSE_ENABLE_KB_PLANNER_RUNNABLE",
         "SURFSENSE_ENABLE_ACTION_LOG",
         "SURFSENSE_ENABLE_REVERT_ROUTE",
         "SURFSENSE_ENABLE_PLUGIN_LOADER",
@@ -57,7 +56,6 @@ def test_defaults_match_shipped_agent_stack(monkeypatch: pytest.MonkeyPatch) ->
     assert flags.enable_llm_tool_selector is False
     assert flags.enable_skills is True
     assert flags.enable_specialized_subagents is True
-    assert flags.enable_kb_planner_runnable is True
     assert flags.enable_action_log is True
     assert flags.enable_revert_route is True
     assert flags.enable_plugin_loader is False
@@ -122,7 +120,6 @@ def test_each_flag_can_be_set_independently(monkeypatch: pytest.MonkeyPatch) ->
         "enable_llm_tool_selector": "SURFSENSE_ENABLE_LLM_TOOL_SELECTOR",
         "enable_skills": "SURFSENSE_ENABLE_SKILLS",
         "enable_specialized_subagents": "SURFSENSE_ENABLE_SPECIALIZED_SUBAGENTS",
-        "enable_kb_planner_runnable": "SURFSENSE_ENABLE_KB_PLANNER_RUNNABLE",
         "enable_action_log": "SURFSENSE_ENABLE_ACTION_LOG",
         "enable_revert_route": "SURFSENSE_ENABLE_REVERT_ROUTE",
         "enable_plugin_loader": "SURFSENSE_ENABLE_PLUGIN_LOADER",

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

@@ -90,8 +90,8 @@ class TestSubstituteInText:
 
 class TestResolveMentions:
     """``resolve_mentions`` resolves chip ids → virtual paths and emits
-    a ``ResolvedMentionSet`` whose id partitions feed
-    ``KnowledgePriorityMiddleware``."""
+    a ``ResolvedMentionSet`` whose id partitions feed the
+    ``search_knowledge_base`` retrieval scope."""
 
     @pytest.mark.asyncio
     async def test_returns_empty_when_no_mentions(self):

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

@@ -161,7 +161,6 @@ class TestInitialFilesystemState:
         assert state["doc_id_by_path"] == {}
         assert state["dirty_paths"] == []
         assert state["dirty_path_tool_calls"] == {}
-        assert state["kb_priority"] == []
         assert state["kb_anon_doc"] is None
         assert state["tree_version"] == 0