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/mention_resolver.py

@@ -74,8 +74,9 @@ class ResolvedMentionSet:
     ``@Project``).
 
     ``mentioned_document_ids`` is an ordered, deduped list consumed by
-    the priority middleware downstream — see
-    ``KnowledgePriorityMiddleware._compute_priority_paths``.
+    the on-demand ``search_knowledge_base`` tool downstream (via
+    ``referenced_document_ids``) to pin @-mentioned docs into the
+    retrieval scope.
     """
 
     mentions: list[ResolvedMention] = field(default_factory=list)
@@ -113,8 +114,8 @@ async def resolve_mentions(
 
     * Legacy clients that haven't migrated to the unified chip list
       still send the id arrays — we treat the union as authoritative.
-    * The id arrays are the canonical input to
-      ``KnowledgePriorityMiddleware`` (via ``SurfSenseContextSchema``);
+    * The id arrays are the canonical input to the retrieval scope
+      (via ``SurfSenseContextSchema`` → ``referenced_document_ids``);
       returning the deduped, validated lists lets the route forward
       them unchanged.
 

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)