feat(coding-agent): complete steer()/followUp() migration

- Update settings-manager with steeringMode/followUpMode (migrates old queueMode)
- Update sdk.ts to use new mode options
- Update settings-selector UI to show both modes
- Add Alt+Enter keybind for follow-up messages
- Update RPC API: steer/follow_up commands, set_steering_mode/set_follow_up_mode
- Update rpc-client with new methods
- Delete dead code: queue-mode-selector.ts
- Update tests for new API
- Update mom/context.ts stubs
- Update web-ui example

packages/web-ui/example/src/main.ts

@@ -346,7 +346,7 @@ const renderApp = () => {
 						onClick: () => {
 							// Demo: Inject custom message (will appear on next agent run)
 							if (agent) {
-								agent.queueMessage(
+								agent.steer(
 									createSystemNotification(
 										"This is a custom message! It appears in the UI but is never sent to the LLM.",
 									),