fix: don't fail convert when input file doesn't exist

When tests are skipped or fail early, the JUnit XML file won't exist.
The convert command should handle this gracefully instead of failing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cosmith

src/commands/convert.ts

@@ -7,8 +7,15 @@ export async function convert({
   from: string;
   to: string;
 }) {
-  // read junit xml file
+  // check if input file exists
   const junitXmlFile = Bun.file(from);
+  if (!(await junitXmlFile.exists())) {
+    console.warn(`Input file not found: ${from}`);
+    console.warn(`Skipping convert (this is normal if tests were skipped or failed early)`);
+    return;
+  }
+
+  // read junit xml file
   const xmlString = await junitXmlFile.text();
 
   // parse junit xml