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_web/app/dashboard/[search_space_id]/user-settings/components/AgentStatusContent.tsx

@@ -125,12 +125,6 @@ const FLAG_GROUPS: FlagGroup[] = [
 				description: "Spin up explore / report_writer / connector_negotiator subagents.",
 				envVar: "SURFSENSE_ENABLE_SPECIALIZED_SUBAGENTS",
 			},
-			{
-				key: "enable_kb_planner_runnable",
-				label: "KB planner runnable",
-				description: "Compile a private planner sub-agent for KB search.",
-				envVar: "SURFSENSE_ENABLE_KB_PLANNER_RUNNABLE",
-			},
 		],
 	},
 	{