Fix non-existent github.run_duration variable in workflow

[Copilot]

The workflow referenced ${{ github.run_duration }}, which doesn't exist in GitHub Actions context and would output empty or error.

Changes

• Add timestamp capture at job start using date +%s stored in START_TIME env var
• Compute elapsed time in PR comment step: ELAPSED_TIME = END_TIME - START_TIME
• Replace ${{ github.run_duration }} with computed ${ELAPSED_TIME}

- name: Record start time
  run: echo "START_TIME=$(date +%s)" >> $GITHUB_ENV

# ... later in workflow ...

- name: Add PR comment with details
  run: |
    END_TIME=$(date +%s)
    ELAPSED_TIME=$((END_TIME - START_TIME))
    
    gh pr comment ... --body "
    - **Processing Time**: ~${ELAPSED_TIME} seconds
    "

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