Update code-scanning-fixer workflow to include critical severity alerts

[Copilot]

Extends the code-scanning-fixer workflow to also detect and fix critical severity alerts, not just high severity.

Changes

• Alert querying: Now makes two API calls (severity: "critical" then severity: "high") and combines results, prioritizing critical alerts
• Workflow prompts: Updated agent instructions, mission goals, and error messages to reference both severity levels
• PR template: Changed hardcoded High severity to [Critical/High] placeholder

Example: Updated Step 2

### 2. List Critical and High Severity Alerts

- First, call `github-list_code_scanning_alerts` with `severity`: "critical"
- Then, call `github-list_code_scanning_alerts` with `severity`: "high"
- Combine results, prioritizing critical alerts over high severity alerts

Original prompt

On code-scanning-fixer.md, Update the code-scanning-fixer.md workflow to also look for critical alerts (in addition to high)

Custom agent used: agentic-workflows
GitHub Agentic Workflows (gh-aw) - Create, debug, and upgrade AI-powered workflows with intelligent prompt routing

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