fix(server): Claude rejects a live change to an option it reads when the query starts

Replacing a started query to apply the option would end its background shells and
tasks. Once the query exists only Fast can change; before the first prompt every option
still applies, because it is read when the query is built.

docs/providers.md

@@ -47,7 +47,8 @@ or without a control.
   there is none; the provider returns the setting to its own default if it can, or throws to
   reject. A key the running session cannot apply (one the provider reads only when it opens) is
   rejected too, so a stored option always describes what runs. Claude reads most options when it
-  starts a query: it applies them from the next turn and rejects them while a turn is running. It
+  starts its query, and replacing a started query would end its background work, so after the
+  first prompt only Fast can change. It
   returns the options it ended up with when they differ from the request. The daemon
   stores the patch and that result in one step after the call resolves, onto the agent's options
   as they are at that moment, so the last write wins. Throwing rejects the whole patch and