Add `datadog-monitor` skill

[niravpatel27]

Summary

Add a datadog-monitor skill that helps set up and manage Datadog monitors, dashboards, and alerts — ensuring services are observable and on-call-ready.

What this skill should do

• Help create monitors for key service metrics (latency, error rate, availability)
• Guide setting up meaningful alert thresholds and notification channels
• Help build dashboards for service health
• Review existing monitors for gaps or noise
• Suggest SLO/SLA definitions

Reference

See existing skills like lfx-backend-builder/SKILL.md for pattern and format.
Each skill lives in its own directory with a SKILL.md file and markdown instructions.

Acceptance Criteria

• datadog-monitor/SKILL.md exists with correct frontmatter
• Skill guides setup of a sample monitor end-to-end