Support guided / follow-up prompting in the desktop client (with steer experience improvements on web)

Summary

When an agent is processing slowly, the client should let the user append additional instructions and have those appended inputs handled with priority, rather than having to wait for the current turn to finish or starting a fresh conversation.

Requested behavior

1. Desktop client should support guided / follow-up prompting.

   • While the agent is busy (e.g. a long-running tool call or slow response), allow the user to append more content.
   • The appended content should be answered according to its priority, so the agent can react to the user's latest input without discarding the in-flight turn.

2. Improve the existing steer experience on the web client.

   • The current steer flow on web should be polished for a smoother UX (clearer affordance, better feedback while waiting, clearer priority semantics).

Motivation

Users occasionally need to redirect or supplement an ongoing request. Today they either have to wait for the current turn to complete or open a new conversation, which loses context. Being able to guide the agent mid-flight makes long-running or slow tasks much more usable.

Acceptance criteria (suggested)

• User can append input while the agent is processing.
• Appended input is visibly acknowledged and handled with the intended priority.
• Web steer experience is easier to discover and gives clear feedback.