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/.env.example

@@ -416,14 +416,6 @@ LANGSMITH_PROJECT=surfsense
 # Skills + subagents
 # SURFSENSE_ENABLE_SKILLS=false
 # SURFSENSE_ENABLE_SPECIALIZED_SUBAGENTS=false
-# SURFSENSE_ENABLE_KB_PLANNER_RUNNABLE=false
-
-# KB retrieval mode (default OFF = lazy). When OFF, the main agent retrieves
-# KB content on demand via the `search_knowledge_base` tool and skips the
-# expensive per-turn pre-injection (planner LLM + embed + hybrid search,
-# ~2.3s); explicit @-mentions are still surfaced cheaply. Set to true to
-# restore the original eager `<priority_documents>` pre-injection.
-# SURFSENSE_ENABLE_KB_PRIORITY_PREINJECTION=false
 
 # Snapshot / revert
 # SURFSENSE_ENABLE_ACTION_LOG=false