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/tests/integration/retriever/test_knowledge_search_date_filters.py

@@ -1,61 +0,0 @@
-"""Integration smoke tests for KB search query/date scoping."""
-
-from __future__ import annotations
-
-from contextlib import asynccontextmanager
-from datetime import UTC, datetime, timedelta
-
-import numpy as np
-import pytest
-
-from app.agents.chat.multi_agent_chat.shared.middleware import knowledge_search as ks
-from app.agents.chat.multi_agent_chat.shared.middleware.knowledge_search import (
-    search_knowledge_base,
-)
-
-from .conftest import DUMMY_EMBEDDING
-
-pytestmark = pytest.mark.integration
-
-
-async def test_search_knowledge_base_applies_date_filters(
-    db_session,
-    seed_date_filtered_docs,
-    monkeypatch,
-):
-    """Date filters should remove older matching documents from scoped KB results."""
-
-    @asynccontextmanager
-    async def fake_shielded_async_session():
-        yield db_session
-
-    monkeypatch.setattr(ks, "shielded_async_session", fake_shielded_async_session)
-    monkeypatch.setattr(
-        ks, "embed_texts", lambda texts: [np.array(DUMMY_EMBEDDING) for _ in texts]
-    )
-
-    space_id = seed_date_filtered_docs["search_space"].id
-    recent_cutoff = datetime.now(UTC) - timedelta(days=30)
-
-    unfiltered_results = await search_knowledge_base(
-        query="ocv meeting decisions",
-        search_space_id=space_id,
-        available_document_types=["FILE"],
-        top_k=10,
-    )
-    filtered_results = await search_knowledge_base(
-        query="ocv meeting decisions",
-        search_space_id=space_id,
-        available_document_types=["FILE"],
-        top_k=10,
-        start_date=recent_cutoff,
-        end_date=datetime.now(UTC),
-    )
-
-    unfiltered_ids = {result["document"]["id"] for result in unfiltered_results}
-    filtered_ids = {result["document"]["id"] for result in filtered_results}
-
-    assert seed_date_filtered_docs["recent_doc"].id in unfiltered_ids
-    assert seed_date_filtered_docs["old_doc"].id in unfiltered_ids
-    assert seed_date_filtered_docs["recent_doc"].id in filtered_ids
-    assert seed_date_filtered_docs["old_doc"].id not in filtered_ids