fix: add required mode input to CodSpeed action

CodSpeed Action v4 requires explicit mode setting. Set to 'walltime'
for standard benchmark measurements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: fengmk2

.github/workflows/codspeed.yml

@@ -33,4 +33,5 @@ jobs:
       - name: Run benchmarks
         uses: CodSpeedHQ/action@v4
         with:
+          mode: walltime
           run: pnpm vitest bench --run