fix(reference-picker): use store subscribe

Follow the state observable API migration by switching reference picker setup from state.subscribe(...) to state.store.subscribe(...).

This matches the refactor that centralized observable helpers under state.store and fixes startup error: attempt to call field 'subscribe' (a nil value).

Verified with: ./run_tests.sh -t tests/unit/reference_picker_spec.lua

Author: oujinsai

lua/opencode/ui/reference_picker.lua

@@ -239,7 +239,7 @@ function M.setup()
     end)
   end
 
-  state.subscribe('messages', function()
+  state.store.subscribe('messages', function()
     M._parse_session_messages()
   end)
 end