Continue promoting the shared agent toolkit out of `new_chat` into the
cross-agent `app/agents/shared` kernel.
- state_reducers.py: clean move (no single-agent importer); all 7 importers
flipped to app.agents.shared.state_reducers.
- context.py: moved to app.agents.shared.context; flipped the multi-agent,
app, automations, chat-flows and monolith importers. A thin re-export shim
remains at new_chat/context.py because the not-yet-retired single-agent
(chat_deepagent) and the new_chat package __init__ still import it; the shim
goes away with the single-agent deletion.
- Updated the stream parity test's annotation normalizer to strip the new
app.agents.shared.context. prefix (SurfSenseContextSchema.__module__ changed
with the move), keeping monolith<->flows signature parity intact.
Behavior-preserving: definitions unchanged; only import paths move. 1219 tests green.
