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).

docker/.env.example

@@ -394,7 +394,6 @@ SURFSENSE_ENABLE_TOOL_CALL_REPAIR=true
 SURFSENSE_ENABLE_BUSY_MUTEX=true
 SURFSENSE_ENABLE_SKILLS=true
 SURFSENSE_ENABLE_SPECIALIZED_SUBAGENTS=true
-SURFSENSE_ENABLE_KB_PLANNER_RUNNABLE=true
 SURFSENSE_ENABLE_ACTION_LOG=true
 SURFSENSE_ENABLE_REVERT_ROUTE=true
 SURFSENSE_ENABLE_PERMISSION=true