Add automated "needs triage" label to new issues

[Copilot]

• Add automated "needs triage" label workflow that triggers on issue creation
• Remove unnecessary dotnet-install.sh entry from .gitignore (was not related to the issue labeling feature)

Changes Made

• Added .github/workflows/auto-triage-label.yml workflow that:
  • Triggers on issue creation (issues: opened)
  • Uses GitHub Actions script to add the "needs triage" label via GitHub REST API
  • Has minimal required permissions (contents: read and issues: write)
  • Follows existing patterns used in other workflows in the repository

Implementation Details

The workflow is simple and focused:

• No checkout is required since we only need to add a label
• Uses the same actions/github-script@v7 action pattern as other workflows
• Minimal permissions following the principle of least privilege
• Does not interfere with existing issue templates or their default labels

This ensures all new issues receive consistent triage labeling without requiring manual intervention from maintainers.

Fixes #1860.

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