chore: Add upstream-prs.json to gitignore

Tracking file for cherry-pick workflow from upstream PRs.

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

Author: Shaun Prince

.gitignore

@@ -5,6 +5,7 @@ CLAUDE.md
 .claude/
 .mcp.json
 
-# Triage tracking (local only)
+# Fork maintenance tracking (local only)
 triaged-issues.json
+upstream-prs.json