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/shared/middleware/compaction.py

@@ -4,7 +4,7 @@ Extends ``SummarizationMiddleware`` with three SurfSense behaviors:
 
 1. A structured summary template (:data:`SURFSENSE_SUMMARY_PROMPT`) instead of
    the base freeform prompt.
-2. Protected SystemMessages (injected hints like ``<priority_documents>``) are
+2. Protected SystemMessages (injected hints like ``<workspace_tree>``) are
    kept verbatim instead of being summarized away.
 3. ``content=None`` is sanitized before ``get_buffer_string`` (some providers
    stream tool-only AIMessages with ``None`` content, which would crash it).
@@ -77,7 +77,6 @@ Respond ONLY with the structured summary. Do not include any text before or afte
 # compaction step happens *before* re-injection in some paths, so we
 # must preserve them verbatim across the cutoff.
 PROTECTED_SYSTEM_PREFIXES: tuple[str, ...] = (
-    "<priority_documents>",  # KnowledgePriorityMiddleware
     "<workspace_tree>",  # KnowledgeTreeMiddleware
     "<file_operation_contract>",  # reserved file-operation contract prefix
     "<user_memory>",  # MemoryInjectionMiddleware