refactor(agents): move feature flags to app/agents/shared/feature_flags (slice 2b)

Promote the agent feature-flag resolver (AgentFeatureFlags / get_flags) out of
`new_chat` into the cross-agent `app/agents/shared` kernel.

feature_flags is a pure leaf consumed across the multi-agent middleware stack,
the chat routes, and tests. Moved it via git mv (content unchanged) and flipped
all 37 importers to app.agents.shared.feature_flags. A thin re-export shim
remains at new_chat/feature_flags.py only for the not-yet-retired single-agent
(chat_deepagent); it goes away with the single-agent deletion.

Behavior-preserving: only import paths change. 1243 tests green.

surfsense_backend/tests/unit/routes/test_revert_turn_route.py

@@ -18,7 +18,7 @@ from unittest.mock import AsyncMock, patch
 
 import pytest
 
-from app.agents.new_chat.feature_flags import AgentFeatureFlags
+from app.agents.shared.feature_flags import AgentFeatureFlags
 from app.routes import agent_revert_route
 from app.services.revert_service import RevertOutcome