agent: Adjust markdown heading sizes (#28759)

Adjust the heading sizes for the Agent Panel so they're not aggressively
huge.

Release Notes:

- N/A

crates/agent/src/active_thread.rs

@@ -23,7 +23,7 @@ use gpui::{
 use language::{Buffer, LanguageRegistry};
 use language_model::{LanguageModelRegistry, LanguageModelToolUseId, Role, StopReason};
 use markdown::parser::{CodeBlockKind, CodeBlockMetadata};
-use markdown::{Markdown, MarkdownElement, MarkdownStyle, ParsedMarkdown};
+use markdown::{HeadingLevelStyles, Markdown, MarkdownElement, MarkdownStyle, ParsedMarkdown};
 use project::ProjectItem as _;
 use rope::Point;
 use settings::{Settings as _, update_settings_file};
@@ -175,11 +175,37 @@ fn default_markdown_style(window: &Window, cx: &App) -> MarkdownStyle {
     });
 
     MarkdownStyle {
-        base_text_style: text_style,
+        base_text_style: text_style.clone(),
         syntax: cx.theme().syntax().clone(),
         selection_background_color: cx.theme().players().local().selection,
         code_block_overflow_x_scroll: true,
         table_overflow_x_scroll: true,
+        heading_level_styles: Some(HeadingLevelStyles {
+            h1: Some(TextStyleRefinement {
+                font_size: Some(rems(1.15).into()),
+                ..Default::default()
+            }),
+            h2: Some(TextStyleRefinement {
+                font_size: Some(rems(1.1).into()),
+                ..Default::default()
+            }),
+            h3: Some(TextStyleRefinement {
+                font_size: Some(rems(1.05).into()),
+                ..Default::default()
+            }),
+            h4: Some(TextStyleRefinement {
+                font_size: Some(rems(1.).into()),
+                ..Default::default()
+            }),
+            h5: Some(TextStyleRefinement {
+                font_size: Some(rems(0.95).into()),
+                ..Default::default()
+            }),
+            h6: Some(TextStyleRefinement {
+                font_size: Some(rems(0.875).into()),
+                ..Default::default()
+            }),
+        }),
         code_block: StyleRefinement {
             padding: EdgesRefinement {
                 top: Some(DefiniteLength::Absolute(AbsoluteLength::Pixels(Pixels(8.)))),