feat(alerts): add configurable threshold alerts with GitHub markdown syntax

[Claude]

Adds a conditional alerts section to the workflow summary that appears when metrics exceed configurable thresholds. Uses GitHub's native markdown alert syntax for better visual prominence.

Changes

Alert Detection (src/post/lib.ts)

• New detectAlerts() function checks three resource types:
  • Memory utilization > threshold (reports first occurrence with step)
  • Sustained CPU > threshold for specified duration (reports all affected steps)
  • Disk usage > threshold (reports first occurrence with step)
• CPU detection tracks all steps during the sustained period, not just when threshold is met

Alert Configuration (action.yml)

• memory_alert_threshold: Default 80%
• cpu_alert_threshold: Default 85%
• cpu_alert_duration: Default 60 seconds
• disk_alert_threshold: Default 90%

Alert Rendering (src/post/renderer.ts)

• Conditionally renders alerts section first in summary (before Workflow Steps)
• Uses GitHub markdown alert syntax with [!WARNING] directive
• Each alert shown as blockquote with emoji icon (⚠️ memory, 🔥 CPU, 💾 disk)

E2E Testing (.github/workflows/test-action.yml)

• New test-alerts-e2e job with lowered thresholds (CPU 10%/10s, Memory 5%, Disk 10%)
• Separate steps trigger each alert type to validate end-to-end functionality

Example Output

### Alerts

> [!WARNING]
> ⚠️ Memory utilization exceeded 80% in step **Memory Heavy Task** (85.3%)
> 🔥 Sustained CPU usage above 85% for more than 60 seconds in steps: **Build**, **Test** (92.7%)