Replace 👀 with ❤️ when response is completed

[ntindle]

Having a finalized state would be awesome so I know its done processing at a glance

[mrT23]

by default we are doing something clearer than the icon - for each persistent tool (/review, /describe, ...) we are sending an updated message.
So I am not sure if there is a great need of also changing the icon

For example
#872 (comment)

[mrT23]

In addition, the default, and by the far the most common, usage of PR-Agent is triggering the /describe, /review, /improve tools automatically when a new PR is opened, so the icons are not relevant there