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.
