Revise and clean up some icons (#35582)

This is really just a small beginning, as there are many other icons to
be revised and cleaned up. Our current set is a bit of a mess in terms
of dimension, spacing, stroke width, and terminology. I'm sure there are
more non-used icons I'm not covering here, too. We'll hopefully tackle
it all soon leading up to 1.0.

Closes https://github.com/zed-industries/zed/issues/35576

Release Notes:

- N/A

crates/agent_ui/src/context_picker/completion_provider.rs

@@ -423,7 +423,7 @@ impl ContextPickerCompletionProvider {
         let icon_for_completion = if recent {
             IconName::HistoryRerun
         } else {
-            IconName::MessageBubbles
+            IconName::Thread
         };
         let new_text = format!("{} ", MentionLink::for_thread(&thread_entry));
         let new_text_len = new_text.len();
@@ -436,7 +436,7 @@ impl ContextPickerCompletionProvider {
             source: project::CompletionSource::Custom,
             icon_path: Some(icon_for_completion.path().into()),
             confirm: Some(confirm_completion_callback(
-                IconName::MessageBubbles.path().into(),
+                IconName::Thread.path().into(),
                 thread_entry.title().clone(),
                 excerpt_id,
                 source_range.start,