citations: consolidate prompts, retire eager path, refresh ADR

Rewrite the main-agent citation contract to a single [n] channel and sync
the orphaned system_prompt_composer surface to match; drop stale
[citation:chunk_id] / <chunk_index> references from dynamic_context and
provider hints. Reuse the shared hybrid search in the deliverables report
(citations omitted for now) and delete the orphaned report KB helper.
Remove the dead eager KnowledgePriorityMiddleware wiring (knowledge_priority
+ stack) and its legacy browse test. Update ADR 0001 to reflect the cutover.

surfsense_backend/tests/integration/google_unification/conftest.py

@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import uuid
-from contextlib import asynccontextmanager
 from datetime import UTC, datetime
 from unittest.mock import MagicMock
 
@@ -227,23 +226,6 @@ def patched_embed(monkeypatch):
     return mock
 
 
-@pytest.fixture
-def patched_shielded_session(async_engine, monkeypatch):
-    """Replace ``shielded_async_session`` in the knowledge_base module
-    with one that yields sessions from the test engine."""
-    test_maker = async_sessionmaker(async_engine, expire_on_commit=False)
-
-    @asynccontextmanager
-    async def _test_shielded():
-        async with test_maker() as session:
-            yield session
-
-    monkeypatch.setattr(
-        "app.agents.chat.multi_agent_chat.subagents.builtins.deliverables.tools.knowledge_base.shielded_async_session",
-        _test_shielded,
-    )
-
-
 # ---------------------------------------------------------------------------
 # Indexer test helpers
 # ---------------------------------------------------------------------------