docs: add conditional execution example using runner.debug

[Copilot]

Adds documentation for conditionally enabling metric collection using the runner.debug context. Teams can leave the action instrumented in workflows but only activate collection on-demand via GitHub's "Enable debug logging" checkbox during re-runs.

Changes

• New section: "Conditional Collection with Debug Mode" positioned after Basic Usage
• Complete example: Shows if: ${{ runner.debug == '1' }} pattern for conditional step execution
• Enablement instructions: Documents the re-run jobs checkbox workflow
• Benefits: No workflow file edits needed to enable/disable collection

Example

steps:
  # Only collect metrics when debug mode is enabled
  - name: Start Workflow Telemetry
    if: ${{ runner.debug == '1' }}
    uses: garbee/runner-resource-usage@v1

  - name: Checkout
    uses: actions/checkout@v6

When investigating resource issues, any team member can enable collection by re-running the workflow with debug logging enabled—no code changes required.

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