ChatSettingsStore keeps each chat's IANA zone in <AgentDir>/telegram/chat-settings.json;
ChatTimeZoneService is the one path for a change (store, live session, scheduler re-evaluation,
reply text). ChatRegistry.TimeZoneFor now reads the store and new sessions start in the chat's zone.
/timezone, the set_timezone tool (ChatSettingsExtension), /status zone line, DEFAULT_TIMEZONE.
The store updates its cache under the write gate so the last file written is the newest state.
Spec 5: the HttpClient registration is named as AddHttpClient<ISpeechToText, OpenAiSpeechToText>() (package Microsoft.Extensions.Http), which is how Program.cs registers it.
