feat: implement burnless status command (closes #4)

[Dinushi94]

What this PR does

Implements burnless status — reads sre.yaml and displays a clean summary of the service's SLO configuration and error budget setup.

Output includes

• Service name and team
• All configured SLOs (name, target %, window, threshold if set)
• Error budget burn rate alerts (severity, rate, window, remediation action)
• Runbooks (name, mode, step count)
• On-call provider and escalation time
• Dashboard provider and auto-generate setting

Design decision

Option B (offline/sre.yaml-only) — works with zero external infrastructure. Live Prometheus querying will be added via --live flag in a follow-up issue.

Testing

• Smoke tested against generated sre.yaml
• make lint clean
• make build clean

Closes #4