Add agent: chat with follow action (experimental) (#31401)

This PR introduces a new `agent: chat with follow` action that
automatically enables "Follow Agent" when submitting a chat message with
`cmd-enter` or `ctrl-enter`. This is experimental. I'm not super
thrilled with the name, but the root action to submit a chat is called
`agent: chat`, so I'm following that wording. I'm also unsure if the
binding feels right or not.

Release Notes:

- Added an `agent: chat with follow` action via `cmd-enter` on macOS and
`ctrl-enter` on Linux

crates/agent/src/agent.rs

@@ -69,6 +69,7 @@ actions!(
         AddContextServer,
         RemoveSelectedThread,
         Chat,
+        ChatWithFollow,
         CycleNextInlineAssist,
         CyclePreviousInlineAssist,
         FocusUp,