Change graph X-axis from time to workflow steps and add final metrics collection

[Copilot]

Graphs displayed time-based X-axis labels, making it difficult to correlate metrics with workflow steps. Additionally, metrics collection stopped before the post action ran, missing the final machine state.

Changes

• X-axis labeling: Replaced time-based labels with workflow step context

  • "Pre-workflow" - metrics before first step
  • "Step Name" - metrics during step execution
  • "Between X and Y" - metrics between consecutive steps
  • "Post-workflow" - metrics after final step

• Final metrics collection: Added collectFinalMetrics() call in post action to capture CPU/memory state after workflow completion

• Simplified error handling: Removed time-based fallback since github-token is required and step markers are always present; now throws explicit error when missing

• Boundary fix: Changed step end boundary check from <= to < so metrics at exact end time don't belong to completed step

Example output:

x-axis "Workflow Steps" ["Pre-workflow","Checkout","Build","Between Build and Test","Test","Post-workflow"]

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