Handle BrokenPipe errors gracefully during git push

When git finishes receiving all the data it needs during a push operation,
it may close the pipe before git-remote-sqlite finishes writing its final
response. This is expected behavior - the push has succeeded from git's
perspective, and we should not treat this as a fatal error.

This was causing CI failures during the repo-db build step when running
`git push --mirror`, where git would close the connection after receiving
all objects but before reading the final "ok" response.

## Test plan

Ran `zig build test` and `zig build repo-db` locally to verify the fix.
The repo-db step now completes successfully without the BrokenPipe error.

Author: chrislloyd

src/transport.zig

@@ -127,8 +127,13 @@ pub const ProtocolHandler = struct {
                 else => return self.fatalError("Command failed: {}\n", .{err}),
             };
 
-            response.format("", .{}, self.out) catch |err| {
-                return self.fatalError("Failed to write response: {}\n", .{err});
+            response.format("", .{}, self.out) catch |err| switch (err) {
+                error.BrokenPipe => {
+                    // Git may close the pipe after receiving all data it needs.
+                    // This is expected behavior and not an error.
+                    break;
+                },
+                else => return self.fatalError("Failed to write response: {}\n", .{err}),
             };
         }
     }