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/app/agents/chat/runtime/path_resolver.py

@@ -4,7 +4,6 @@ This module is the single source of truth for mapping ``Document`` rows to
 virtual paths under ``/documents/`` and back. It is used by:
 
 * :class:`KnowledgeTreeMiddleware` (rendering the workspace tree)
-* :class:`KnowledgePriorityMiddleware` (computing priority paths)
 * :class:`KBPostgresBackend` (``als_info`` / ``aread`` / move operations)
 * :class:`KnowledgeBasePersistenceMiddleware` (resolving moves and creates)