Refine branch item spacing and header-content merging

Improve visual cohesion by adjusting margins and padding in BranchItem:
- Normalize margins on toggle header
- Add conditional spacing based on shouldMergeHeader flag
- Use negative margin to merge header with content seamlessly
- Adjust padding to eliminate gaps when merged

This creates a more polished appearance for collapsed/expanded states.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: brandonkachen

cli/src/components/branch-item.tsx

@@ -162,6 +162,8 @@ export const BranchItem = ({
               paddingRight: 4,
               paddingTop: 0,
               paddingBottom: 0,
+              marginTop: 0,
+              marginBottom: 0,
             }}
             onMouseDown={onToggle}
           >
@@ -200,7 +202,14 @@ export const BranchItem = ({
             </text>
           )}
           {!isCollapsed && content && (
-            <box style={{ flexDirection: 'column', gap: 0, marginTop: 1 }}>
+            <box
+              style={{
+                flexDirection: 'column',
+                gap: 0,
+                marginTop: shouldMergeHeader ? -1 : 1,
+                marginBottom: shouldMergeHeader ? 0 : 0,
+              }}
+            >
               <box
                 style={{
                   flexDirection: 'column',
@@ -211,7 +220,7 @@ export const BranchItem = ({
                   borderColor: cornerColor,
                   paddingLeft: 1,
                   paddingRight: 2,
-                  paddingTop: 1,
+                  paddingTop: shouldMergeHeader ? 0 : 1,
                   paddingBottom: 1,
                 }}
               >
@@ -228,6 +237,7 @@ export const BranchItem = ({
                       paddingBottom: 0,
                       marginBottom: 1,
                       width: '100%',
+                      marginTop: 0,
                     }}
                     onMouseDown={onToggle}
                   >