feat(app): default to steering when sending into a running agent

Steer is the better default for Enter while an agent is running, but
flipping DEFAULT_CLIENT_SETTINGS alone would only reach new installs.
Client defaults are materialized into storage — the first load writes the
whole default blob and every save re-stamps it — so every existing install
already has "interrupt" on disk, and a stored "interrupt" cannot be told
apart from one the user actually chose. A one-time migration flips them
all; picking "interrupt" afterwards sticks.

The applied-migration marker sits in its own storage key rather than in the
settings blob: StoredAppSettingsSchema is strict and readValidatedJson
clears the value when parsing fails, so an unknown field there would wipe
every setting for anyone who downgrades.

migrateAppSettings owns both of its writes so the settings blob always
lands before the marker. A failed marker write leaves the migration to
re-run harmlessly; a failed settings write must leave the marker unwritten,
or the migration is lost and the user keeps "interrupt" for good.

packages/app/src/composer/input/input.tsx

@@ -147,9 +147,10 @@ export interface MessageInputProps {
   voiceAgentId?: string;
   /** When true and there's sendable content, calls onQueue instead of onSubmit */
   isAgentRunning?: boolean;
-  /** Controls what the default send action (Enter, send button, dictation) does
-   *  when the agent is running. "interrupt" sends immediately, "queue" queues. */
-  defaultSendBehavior?: "interrupt" | "steer" | "queue";
+  /** Controls what the default send action (Enter, send button, dictation) does when the agent is
+   *  running. "interrupt" and "steer" send immediately, "queue" queues. Required so the default
+   *  lives only in DEFAULT_CLIENT_SETTINGS. */
+  defaultSendBehavior: "interrupt" | "steer" | "queue";
   /** Callback for queue button when agent is running */
   onQueue?: (payload: MessagePayload) => void;
   /** Optional handler used when submit button is in loading state. */
@@ -1132,7 +1133,7 @@ function resolveMessageInputProps(props: MessageInputProps): ResolvedMessageInpu
     voiceServerId: props.voiceServerId,
     voiceAgentId: props.voiceAgentId,
     isAgentRunning: props.isAgentRunning ?? false,
-    defaultSendBehavior: props.defaultSendBehavior ?? "interrupt",
+    defaultSendBehavior: props.defaultSendBehavior,
     onQueue: props.onQueue,
     onSubmitLoadingPress: props.onSubmitLoadingPress,
     onKeyPressCallback: props.onKeyPress,