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/lib/apis/agent-flags-api.service.ts

@@ -19,7 +19,6 @@ const AgentFeatureFlagsSchema = z.object({
 
 	enable_skills: z.boolean(),
 	enable_specialized_subagents: z.boolean(),
-	enable_kb_planner_runnable: z.boolean(),
 
 	enable_action_log: z.boolean(),
 	enable_revert_route: z.boolean(),