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/src/context.rs

@@ -42,8 +42,8 @@ impl ContextKind {
             ContextKind::Symbol => IconName::Code,
             ContextKind::Selection => IconName::Context,
             ContextKind::FetchedUrl => IconName::Globe,
-            ContextKind::Thread => IconName::MessageBubbles,
-            ContextKind::TextThread => IconName::MessageBubbles,
+            ContextKind::Thread => IconName::Thread,
+            ContextKind::TextThread => IconName::TextThread,
             ContextKind::Rules => RULES_ICON,
             ContextKind::Image => IconName::Image,
         }