Display performance graphs inline in Actions summary

[Copilot]

Continues SemiAnalysisAI#205 by uploading PNG graphs to an images branch and embedding them directly in the GitHub Actions summary instead of requiring artifact downloads.

Changes

• Permissions: Added contents: write to enable pushing to images branch
• Upload step: Implements austenstone/file-upload pattern - commits PNGs to orphan images branch, constructs raw.githubusercontent.com URLs
• Summary generation: Markdown with embedded images grouped by metric type (Interactivity/E2E Latency) and model family

Implementation

- name: Upload and display images
  run: |
    # Commit PNGs to images branch
    git checkout -B images origin/images || git checkout --orphan images
    git add *.png && git commit -m "..." && git push -f origin images
    
    # Generate summary with raw GitHub URLs
    echo "![graph](https://raw.githubusercontent.com/${repo}/refs/heads/images/graph.png)" \
      >> $GITHUB_STEP_SUMMARY

Handles dynamic file count (model families vary), preserves existing artifact upload for archival.

Original prompt

Continue https://github.com/InferenceMAX/InferenceMAX/pull/205

use austenstone/file-upload
Example:

      - id: upload-image
        uses: austenstone/file-upload@v1
        with:
          path: output.png
          
      - name: Add image to job summary
        run: |
          echo "![Generated Image]($IMAGE_URL)" >> $GITHUB_STEP_SUMMARY
        env:
          IMAGE_URL: ${{ steps.upload-image.outputs.url }}

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